# What do I use to seal MDF?



## Sleeper

I just built my very first project with MDF and I’m afraid to paint it. Years ago I painted something and I can’t remember now what kind of material it was, but it swelled up like a sponge and it might very well have been MDF.


I searched for this topic and found one in the Wood Finishing forums that nobody responded to so I’m posting here in hopes someone will respond since there are more reading posts here.


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## Corbin3388

I'd use oil primer and oil based paint. It is ONLY suitable for interior use.

Sent from my iPhone using Wood Forum


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## Sleeper

Corbin3388 said:


> I'd use oil primer and oil based paint. It is ONLY suitable for interior use.
> 
> Sent from my iPhone using Wood Forum


I think I may have a can of oil primer. It's pretty old maybe 3 years, but never been opened. I guess I will not know until I open it. :smile:


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## firemedic

I'd suggest:

Dewaxed shellac sealer, lacquer sealer or thinned down yellow glue... Sand, sand, sand... Any paint you chose!

Oil based sealers would work too though.

Good luck!

~tom "Ignorance is not a lack of intelligence - it's a lack of know-how"


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## jschaben

Sleeper said:


> I just built my very first project with MDF and I’m afraid to paint it. Years ago I painted something and I can’t remember now what kind of material it was, but it swelled up like a sponge and it might very well have been MDF.
> 
> 
> I searched for this topic and found one in the Wood Finishing forums that nobody responded to so I’m posting here in hopes someone will respond since there are more reading posts here.


 
Hi - I like sealing with BLO as it also makes the surface hard and slick. I've also used oil based primer and BIN (a shellac based primer). I would think it would handle a good bonding latex primer although I can't recall trying that. I'd stay away from water based PVA drywall primers though. :smile:


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## Joeslake

make an epoxy batch dilluted with alcohol so it is much thinner. Then brush it on. It'll leave a hardened surface.


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## MastersHand

Joeslake said:


> make an epoxy batch dilluted with alcohol so it is much thinner. Then brush it on. It'll leave a hardened surface.


+1 I have used West Systems for this it's sands up nice and painting is a breeze


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## Shop Dad

I haven't had a chance to try this yet but will: In the back of the "Tune Up Your Tools" booklet from Fine Woodworking they have a note on sealing MDF. The edges are more absorbent than the faces and the recommendation is to seal them with glue size - 50/50 mix of PVA glue/water. Once dry lightly sand then seal the whole thing with shellac.


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## Sleeper

Thanks everybody, I’m starting to think that I made a huge mistake using this stuff. There may be one problem in that I have already built it into a table saw sled with T-slots, is that going to be an issue? I'm mentioning it because of the sanding statements. 

Also I’ve seen “BLO” mentioned before, but I have no idea as to what that is.

Anyway this is what I’m using it on:










Do you foresee any problems?


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## jschaben

Joeslake said:


> make an epoxy batch dilluted with alcohol so it is much thinner. Then brush it on. It'll leave a hardened surface.


Sounds like it would work great. $$$$/sq ft. about?:detective:


Johnny - BLO = Boiled Linseed Oil. Should work fine on your sled. Follow the cautions on the can, stuff can be pretty flammible.


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## Steve Neul

It would depend on what finish you are going to topcoat it with. The face of MDF should be sanded before finishing because the resins will react with the finish. You could use a latex primer and topcoat it with latex paint however it would take a lot of primer to seal any edges. You could prime it with kilz original primer and topcoat it with either latex or oil based paint. Then you could use a lacquer primer and topcoat it with lacquer paint. For that I like Bushwacker primer from Sherwin Williams. MDF can also be stained and finished with clear polyurethane or clear lacquer. 

MDF will swell up if exposed to a significant amount of water but you should not have that problem with finishes.


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## Sleeper

I don’t care so much of what it looks like so I probably will pass on any kind of stain. I’m just more concerned about preserving it so it doesn’t mushroom up. 

The only reason that I used it in the 1st place was that I saw so many using it for jigs and sleds. Plus it was also convenient that it had a perfectly flat and smooth surface. I’m now concerned that the edges around the slots might be a problem if they start to pop up right in the middle of my sled. 

I’m not sure yet what I’ll do. I have some clear polyurethane and I also have a scrap piece that I guess I could experiment with some of the sealers that I already have.


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## jschaben

Nothing wrong with MDF, I use it on jigs all the time and made my drill press table from it. Like I said, the Linseed oil soaks in and hardens it. I usually end up with 4 or 5 applications of it. Just keep applying it, give it 15 or 20 minutes to soak up that application and do it again till it won't take any more. Then just wipe off any excess and let it dry, may take a day or so to completely dry but will continue to cure and harden.
Well sealed MDF isn't near the issue that I see bantered about. I wouldn't be using it for exterior applications but some places in Europe use it for exterior shutters.:smile:


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## Jim West Pa

If it's only gonna be used on yer TS and has no chance of bein exposed to moistures then why put any finish at all on it ?
You obviously built it so you can make accurate mitres. Would there not be a chance of a finish interferin with this accuracey ?

I've built jigs and fences with MDF, never put any finish on 'em and never had any issues.


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## Sleeper

jschaben said:


> Nothing wrong with MDF, I use it on jigs all the time and made my drill press table from it. Like I said, the Linseed oil soaks in and hardens it. I usually end up with 4 or 5 applications of it. Just keep applying it, give it 15 or 20 minutes to soak up that application and do it again till it won't take any more. Then just wipe off any excess and let it dry, may take a day or so to completely dry but will continue to cure and harden.
> Well sealed MDF isn't near the issue that I see bantered about. I wouldn't be using it for exterior applications but some places in Europe use it for exterior shutters.:smile:


 Well I hope I can find some other uses for it because I had to buy a full sheet and have a 4x6 piece left over.



Jim West Pa said:


> If it's only gonna be used on yer TS and has no chance of bein exposed to moistures then why put any finish at all on it ?
> You obviously built it so you can make accurate mitres. Would there not be a chance of a finish interferin with this accuracey ?
> 
> I've built jigs and fences with MDF, never put any finish on 'em and never had any issues.


 I can’t guaranty that there will never be any water, because my table saw is up against my roll-up garage door and sometimes when it’s raining hard and someone opens the door there are drips from the door. Plus I have spilled my coffee on occasion. :shifty:










Right now I'm trying to find a place to store it when not using it and it looks like its going to have to be on its side on the floor.

Anyway I went through all my paint supplies and I think I’m going to have to buy something. I’m going to take a look at that linseed oil. The one reason that I’m not going to use the oil base primer is that it’s too thick and I want something that is thin enough to get into the T-slots and perhaps the blade slot if that's possible.


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## Shop Dad

FYI - The FWW article was about finishing jigs like this. The glue size and shellac should provide some protection - though I wouldn't let it sit in a puddle with any finish. Maybe put some blocks under it if it's on the floor. You can run some wax over the runners to keep them sliding nicely. 

Look forward to seeing how you work this out. A sled is on my build list for the spring. :smile:


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## Sleeper

I picked up a can of Boiled Linseed Oil from Wal-Mart and now I remember this stuff. I also remember why I never used it again. It can cause spontaneous combustion and is very dangerous. I knew someone who had a trash can fire from this stuff.

Anyway I tried some on the side edge where I can cut it off if I have to and so far it looks ok except its going to need a little sanding. I’m not sure if I like this stuff, but it says 24 hrs to dry so I will not know until tonight.


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## Sleeper

Shop Dad said:


> ....... Maybe put some blocks under it if it's on the floor. You can run some wax over the runners to keep them sliding nicely.........


I need to do this because it does stick a little, but i just don't know what wax to get


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## Shop Dad

If you have paste wax try that. Otherwise pick up some paraffin/canning wax at the grocery store. It's pretty cheap.


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## Sleeper

Shop Dad said:


> If you have paste wax try that. Otherwise pick up some paraffin/canning wax at the grocery store. It's pretty cheap.


I have car wax :icon_confused:


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## Shop Dad

Sleeper said:


> I have car wax :icon_confused:


Most car wax has silicone which can affect finishing as I learned from the forum here. Avoid that.


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## jschaben

Sleeper said:


> I have car wax :icon_confused:


Car wax generally contain silicone which will mess with finishes. I use Johnsons paste wax. I also use a mixture of mineral oil and parafin as a lubricant and sealer for some things.


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## Cliff

Sleeper said:


> I just built my very first project with MDF


I don't know whether congratulations or curses is in order. 

What did you build?
Why do you want to paint it? 
What function is the paint going to serve? 

Important questions I should think.


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## cabinetman

Cliff said:


> I don't know whether congratulations or curses is in order.
> 
> What did you build?
> Why do you want to paint it?
> What function is the paint going to serve?
> 
> Important questions I should think.


Some projects are painted projects. MDF paints up very well.










 







.


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## Icutone2

I seal MDF with water base Sand Seal, also use drywall compound to level low spots before painting. Just my 2 cents.
Lee


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## jschaben

Crosscut sled, with t-slot adjustments. Just doesn't cry out for paint to me. Paint get in the t-slots and mess with the adjustability, unless a very hard, gloss paint it will increase the drag..
I'd finish that with linseed oil followed with a good coat of paste wax. done deal:smile:


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## Sleeper

Cliff said:


> I don't know whether congratulations or curses is in order.
> 
> What did you build?
> Why do you want to paint it?
> What function is the paint going to serve?
> 
> Important questions I should think.


 Well as I mentioned before, I only used the stuff because and I’m guessing about 70% of the jigs I saw were MDF and I didn’t want to waste my good plywood on something that I did not care what it looked like. Also like I said before it was smooth, flat and straight so I thought it would make an accurate jig.

What I didn’t think of was that it was subject to absorbing moisture and affecting its accuracy. So I wanted to seal it and I read not to use latex paint because it will mushroom the edges and surface. I can’t have raised edges right in the middle of my flat surface as I mentioned before.

So I just happened to see the BLO at Wal-Mart during a trip with the wife and decided to give it a try. As it turns out that I have used this stuff 30 years ago, not on MDF but on an exposed knotty pine ceiling which in itself was pure misery to do, because of the mess and the worry of fire. At the time I thought I would never use this stuff again.

If the BLO works out then I want to use it up just to get rid of the stuff and I have another 4x6’ piece of MDF to get rid of. So it looks like I may be making more jigs out of this stuff.


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## Sleeper

I coated the underside a little while ago and I noticed this when I just checked on it. 

It does not look good and I'm very disapointed. I coated the edge last night and it did not look like this. I may have to build a new sled if the rest does this.


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## Sleeper

Just to show what is going on, the top layer is peeling up as the BLO dries.


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## Jim6620

Use a small round over bit like 1/32 radius then mixup west system epoxy (slow set) and use a foam brush to flood it on. Make sure the MDF and the epoxy are the same temp and above 70 degrees. That will sink in and turn the edges to stone!! It works great for painted window seats with bullnose.


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## jschaben

Sleeper said:


> Just to show what is going on, the top layer is peeling up as the BLO dries.


yikes, I've never had it do that on me. Jim's post did remind me though, I've always rounded the edges over first... no particular reason at the time, just liked the way it looked and felt. :huh:


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## Sleeper

Jim6620 said:


> Use a small round over bit like 1/32 radius then mixup west system epoxy (slow set) and use a foam brush to flood it on. Make sure the MDF and the epoxy are the same temp and above 70 degrees. That will sink in and turn the edges to stone!! It works great for painted window seats with bullnose.


 Thanks I have try that after everything dries. 

It’s spreading all the way around the edge and I sure am glad that I did the bottom first. I’m done with BLO and MDF


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## jschaben

I'm curious as to what is going on there. This is a drill press table I made from MDF a little over two years ago. 4 or 5 applications of BLO (until it wouldn't take any more) and installed the t-tracks after. Hard as a rock and slick.. even spilled some coffee on it once and it just beaded up.
All I can think of is that it had already started absorbing moisture between the time you cut it up and got the finish applied, especially if it was left out in the garage or someplace where humidity could get to it.


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## Sleeper

jschaben said:


> I'm curious as to what is going on there. This is a drill press table I made from MDF a little over two years ago. 4 or 5 applications of BLO (until it wouldn't take any more) and installed the t-tracks after. Hard as a rock and slick.. even spilled some coffee on it once and it just beaded up.
> All I can think of is that it had already started absorbing moisture between the time you cut it up and got the finish applied, especially if it was left out in the garage or someplace where humidity could get to it.


 Nice drill press table, I need to make one of those myself.

Anyway I just don’t know, maybe it’s just the brand of MDF I have no idea. The only other thing I can think of is that I was going to wipe the BLO on with a rag, but I got worried about the possibility of spontaneous combustion after i finished and decided to use a brush so it’s possible that I might have got it on too wet.

Mine looks a lot darker with only one coat


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## jschaben

Sleeper said:


> Nice drill press table, I need to make one of those myself.
> 
> Anyway I just don’t know, maybe it’s just the brand of MDF I have no idea. The only other thing I can think of is that I was going to wipe the BLO on with a rag, but I got worried about the possibility of spontaneous combustion after i finished and decided to use a brush so it’s possible that I might have got it on too wet.


Hi Johnny - you can't get it to wet, at least not on the first coat. Usually it soaks the stuff up as fast as I can put it on. Just keep brushing it on until it starts to puddle, just let the puddles sit for 15 to 20 minutes. If the puddles are gone after 15 minutes, add another coat until there are puddles left after 15 minutes. Then wipe off the excess with a rag. Spontaneous combustion occurs when the oil soaked towels are place in a pile or heaped together where they can get hot enough to ignite. I spread them outside in the drive way until they dry out then throw them away. You don't want a heap of the things laying around or in a container where there is limited air circulation. The red disposer cans you see are (supposed to be) air tight. Not necessary if they are allowed to dry before disposal.


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## Cliff

cabinetman said:


> Some projects are painted projects. MDF paints up very well.


Yah this is true. I'm curios about those factors I mentioned because (a) not all paints are equal and (b) the OP wants to "seal" it without informing what he means by sealing or what he wants to seal against or whether he wants this seal to serve any other function.


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## Sleeper

Cliff said:


> Yah this is true. I'm curios about those factors I mentioned because (a) not all paints are equal and (b) the OP wants to "seal" it without informing what he means by sealing or what he wants to seal against or whether he wants this seal to serve any other function.


 Well Cliff, I’ve read the contrary about paint and I also read tto never use latex paints unless it is sealed prior to painting with oil based primer, BLO or varnish. I just walked in from working all day and I’m too tired to find the articles on the internet, but you can see them by just searching “painting MDF”. I believe one was from the manufacture of MDF.

I don’t really care about paint anyway, because its a tool and I don't care what it looks like as long as its functional and accurate. I just don’t want it to swell, deform, warp, split or whatever from moisture. 

Anyway, I had to pick up a new roundover bit on my way home from work because the one I had was a cheap one that I bought in 1971 without a barring. It was cheaper to buy a set of three then a single and I will give it a try after dinner. I’m undecided if I will use anymore BLO after I remove the peeling around the edge.


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## Sleeper

I decided to go ahead and do the whole Sled except that this time I spread it on with a brush and immediately wiped it off with a rag. It came out a lot better but I did have some rough raised edges around the slots. I sanded them down flush with the table and I’m happy with it.

I know I said that I didn’t care if they were painted, but now that it’s so dark I may paint just the top white so I can see any pencil marks I may use.


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## Burt

I made my benchtop out of 3/4 mdf with a few T-tracks sunk in lenth of bench. I used Danish oil to seal the surface a few years ago with no problems. It soaks into suface and hardens it.


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## Sleeper

Just finished my brand new Drill Press Table & Fence with BLO and I think it came out OK. I wiped it with a rag right after applying it and also I did the top and bottom first then the sides after it dried for a couple of hours. 

I just did the Doug Fir fence and I really had to wipe it down because it didn’t absorb at all and started to run.


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## jschaben

Sleeper said:


> Just finished my brand new Drill Press Table & Fence with BLO and I think it came out OK. I wiped it with a rag right after applying it and also I did the top and bottom first then the sides after it dried for a couple of hours.
> 
> I just did the Doug Fir fence and I really had to wipe it down because it didn’t absorb at all and started to run.


Hi Johnny - yeah!!! That looks more like it :thumbsup:


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## Sleeper

jschaben said:


> Hi Johnny - yeah!!! That looks more like it :thumbsup:


 Thanks I can see now from the photo that I need to do the edges again tomorrow


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## jschaben

Sleeper said:


> Thanks I can see now from the photo that I need to do the edges again tomorrow


edges will likely take at least one more coat. The stuff is pretty thirsty. 
The more I think about it, the more convinced I am that your other project started soaking up humidity before you were able to get the finish on. I've never seen MDF come apart like that unless it's been exposed. I bought a piece of 2'x4' x 1/4" from Home Depot once and one pass through the saw and it came apart into 1/8" sheets.


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## Sleeper

jschaben said:


> ...
> The more I think about it, the more convinced I am that your other project started soaking up humidity before you were able to get the finish on. I've never seen MDF come apart like that unless it's been exposed. I bought a piece of 2'x4' x 1/4" from Home Depot once and one pass through the saw and it came apart into 1/8" sheets.


 Well it very well could have been. I bought it from Home Depot and had to buy a 4x8 sheet. The TS Sled was the very first piece that I cut off the sheet on the end and the DP table was in the middle of the sheet. The MDF was on its side on the floor at Home Depot and I didn’t even see it until I asked for it. There was only about 3 or 4 sheets and I guess it was taking up too much space laying flat on the shelves.


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## Cliff

Sleeper said:


> Well as I mentioned before, I only used the stuff because and I’m guessing about 70% of the jigs


 Jigs are small. I've sealed the edges of MFD for shop jigs with West epoxy using the slow hardener to let it soak in. It works rather well for small surface area where wear would be an issue. If I were doing a large area just to cover it and get minimal protection I'd paint with oil. I'd not use BLO on anything that the word "seal" was part of the equation. BLO is organic and is not hydrophobic. Moisture sails right through it.


> latex paint because it will mushroom the edges


 Yipper it'll cause all manner of problems. There are cheaper epoxies out there than West. Google it. There's Epoxysystems and a slew of other chemical formulators who'll do business cheaper than west. West is so spendy not because it's better than others but because it is sold to the marine and aircraft i industry and as such it carries a whole heap of liability when planes crash or boats founder. The marketing they do doesn't help lower costs either. When you buy West you are paying to cover their liability in those areas and for all the pretty adverts.


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