Best methods for finishing MDF

[davepellegrene]

Since there has been so much talk about seams showing through MDF. I am going to do some experimenting with different methods used to seal it. I am going to put together some test samples to try different techniques side by side to see what works the best, easiest to apply and the least expensive method.

From what I have read seems most everybody agrees the inside should be sealed as well. To keep it simple I am going to glue up a 90 degree joint and seal all sides.

The methods I have so far will be:
1. Tight bond glue thinned 60/40. I have not done this method, so chime in on this one. I figure at least two coats more on the edges as it soaks in.
http://www.titebond.com/DetermineTB....uctIntroTB.asp

2. Bin 123 white pigmented shellac. Two coats more on the edges until it stops soaking in.
http://www.zinsser.com/product_detail.asp?productid=10

3. Waterborne automotive primer. Two coats more on edges until it stops soaking in.
http://yhst-13811118617756.stores.ya...2hywaprga.html

4. Fiberglass resin. Not sure on coats. Haven't tried this method yet. Probably two with more on the edges.

Does anybody want to add a method to the list they think works well?
I am also going to try baking the moisture out of the MDF first to see if that makes a difference.
I am going to stop at the local granite company to see if they have any scrap pieces of moisture resistant MDF. I have seen a lot of this being used as backer for granite, marble and solid surfaces in kitchens and bathrooms.

Dave

[zach_t]

There was a very large thread over at DIYAUDIO.COM on this, I will try to find it.

Here are my thoughts....

1. Cover box with bondo or wood filler. Sand smooth.
2. Seal exposed MDF with Sanding Sealer, couple coats
3. Cover the entire box in Shellac
4. Then cover with automotive primer.
5. Top Coat.

[monoman]

On my last build I used clear gloss polyurethane finish to seal the MDF, as I had a half empty can I wanted to use up. It seemed to do the job okay, but I am still new at this.
Has anyone else used clear poly to seal MDF? I am curious what other people here think of this.

[Wolf]

I rub in plain 'ol white Elmer's glue, undiluted. WITH MY FINGERS!! I use a line down the edge, and rub in until I've covered the whole edge, adding more if necessary. Just enough for coverage, or an entirely wet feel before dry.
When dry, sand with 150/250g, proceed to automotive primer...

Later,
Wolf

[davepellegrene]

I rub in plain 'ol white Elmer's glue, undiluted. WITH MY FINGERS!! I use a line down the edge, and rub in until I've covered the whole edge, adding more if necessary. Just enough for coverage, or an entirely wet feel before dry.
When dry, sand with 150/250g, proceed to automotive primer...

Later,
Wolf

Thats the way I was taught to put on tongue oil. The heat from your hands was suppose to help work it into the wood. I was also thinking about using high gloss Krylon as one of the top coats, since it is an easier method for most to apply.
Dave

[davepellegrene]

There was a very large thread over at DIYAUDIO.COM on this, I will try to find it.

Here are my thoughts....

1. Cover box with bondo or wood filler. Sand smooth.
2. Seal exposed MDF with Sanding Sealer, couple coats
3. Cover the entire box in Shellac
4. Then cover with automotive primer.
5. Top Coat.

I will work in this method. I was thinking about using bond to see it it would bridge the end grain enough to hide the seam anyway. I have all the suppies to do this one also.
Dave

[davepellegrene]

On my last build I used clear gloss polyurethane finish to seal the MDF, as I had a half empty can I wanted to use up. It seemed to do the job okay, but I am still new at this.
Has anyone else used clear poly to seal MDF? I am curious what other people here think of this.

It may be worth a try. Maybe use a sanding sealer, then poly, then paint.
Dave

[ocdSCHACK]

There's a thread somewhere and they swear by some water based filler primer. I think it's meant for MDF and sold for the speaker industry. I know it's white and can't remember what the stuff what was, but it didn't look like mass-marketed junk you usually end up buying at the store. Someone in this forum knows of the stuff of which I speak. Where are they?

[lunchmoney]

Thanks so much for doing this!

I would add shellac into the mix. Although I tried it and didn't like it ("pilled up" upon sanding), many swear by it.

Another I've been curious about is straight elmer's white glue... I think this is Wolf's favorite... although I suspect it would be too thick to get a nice even coat.

Variables that are going to make your evaluation tricky (among many others)... not trying to discourage you, but I'm wondering how you'll factor them into your results:

1. Number of coats
2. How long you wait after sealing, before sanding
3. Humidity

DO NOT do what I just tried, which is a 2:1 mix of Elmer's wood glue and water... too thick to spread smoothly, resulted in some hardened "drip" shapes... sanding was a bitch, but I think I'll be ok... hopefully the 5 thick coats of high build primer will compensate

[lunchmoney]

In all cases, it's not about sealing the surface for paint... that's easy.

It's about whether ANY technique can keep seams from emerging. I'm skeptical, as you're only sealing the top millimeter or so.

Any moisture already in the mdf prior to sealing WILL expand.

I suspect that some method of baking the mdf BEFORE sealing would be required.

[jerryanderson]

On my last build I used clear gloss polyurethane finish to seal the MDF, as I had a half empty can I wanted to use up. It seemed to do the job okay, but I am still new at this.
Has anyone else used clear poly to seal MDF? I am curious what other people here think of this.

My experience has showed that it takes more applications to seal up edges with poly! Keeps soaking in! For me, a couple coats of 60/40 TiteBond over the entire project~dried/lightly sanded smooth, then a coat or two of clear poly, repeat sanding.

[lunchmoney]

My experience has showed that it takes more applications to seal up edges with poly! Keeps soaking in! For me, a couple coats of 60/40 TiteBond over the entire project~dried/lightly sanded smooth, then a coat or two of clear poly, repeat sanding.

That is exactly the technique I use... but eventually the seams still emerge slightly.

[learn2turn]

What about taking a different approach. Suppose you sand and seal and then coat with something adhesive like paintable wall paper. I supposed if you are going to paper, you might think it just as easy to use a finish laminate but if you did want a pained finish, using a paintable paper might prevent seams from showing.

[williamrschneider]

I'm a skeptic about any easy-to-use coating being able to prevent seams from showing. I've veneered cabinets that were sanded dead smooth before application, and eventually the seams telegraphed through 0.025" thick veneer with a glossy finish. It's irritating after having spent so much time prepping a joined surface to be dead smooth to have imperfections appear later.

I've started adding TWO layers of veneer on top surfaces to minimize the eventual appearance of the but* joint seams. I'll report back after finishing one of these projects and waiting a couple months.

Mitered corners are probably the solution, but what a pain!

[Dirk]

I rub in plain 'ol white Elmer's glue, undiluted. WITH MY FINGERS!! I use a line down the edge, and rub in until I've covered the whole edge, adding more if necessary. Just enough for coverage, or an entirely wet feel before dry.
When dry, sand with 150/250g, proceed to automotive primer...

Later,
Wolf

And then peel the tried glue off your finger like you were six years old again.

I dilute, personally. A couple coats with a paint brush. I also do one coat of the whole box. I've noticed that, if I'm doing paint, spots on the front baffle that got a little glue "overrun" finish differently than spots that did not (even after a sanding of the whole baffle). Usually, those spots take the paint better anyway.

[LouC]

I've seen bu*t joints on MDF show up two years after they were perfectly finished by a master craftsman (not me!) who refinished pianos for a living. (I watched Herb turn cheap cherry veneer patches into extinct 1870's era rosewood with brushes, dyes & stains. That 2 year restoration went for $300,000)
Unless you live somewhere with constant heat and humidity levels, joints lines are probably inevitable. Better quality MDF has a hardened outer layer and softer core material so exposing the edge is an almost impossible battle. It's why I quit trying to paint and went to veneer and/or Duratex.

His recommended solutions to me:

• veneer over the face with joints with a tight grain wood (maple,yellow pine,...) then fill the pores and prep the veneer for paint.
• Change your colors at a joint
• Miter or bevel instead of roundover
• Make the joints a feature with a groove painted in contrasting color
• Use a flexible clear finish that dissolves prior coats & crosslinks --in case you want to refinish down the road (lacquer, acrylic, not polyurethane)

[Jim Holtz]

I've tried a number of different methods of sealing the seams and the only one I've found that is permanent is 1/8" MDF. I cap the ends/surfaces with all of the seams using 1/8" MDF and then you just have a 1/8" seam that is on an edge to worry about.

BTW, bondo is the best solution other than 1/8" MDF I've found for seams.

Jim

[kirk78h]

If I was covering a seam that I never wanted to show, I would use a Dynatron bumper repair kit. I've used this for several custom automotive interior fabrications, and I've never seen it crack. Other interior mods I've seen that used bondo almost always fail because it sets up too hard; and has no 'give'. Dynatron remains flexible enough to withstand some expansion. I've never used it on MDF, so I don't know if it would have proper adhesion directly. I have seen other people start with MDF mounting rings, then add epoxy putty for molding / large gaps; then finish with Dynatron. Just my $.02.

Kirk

[djg]

If you are really energetic, you might try a layer of light fiberglass cloth with epoxy resin. Homebuilt aircraft use this over foam cores, model aircraft use it over veneer covered foam wing cores, and stitch and glue homebuilt boats use it over plywood.

[davepellegrene]

If I was covering a seam that I never wanted to show, I would use a Dynatron bumper repair kit. I've used this for several custom automotive interior fabrications, and I've never seen it crack. Other interior mods I've seen that used bondo almost always fail because it sets up too hard; and has no 'give'. Dynatron remains flexible enough to withstand some expansion. I've never used it on MDF, so I don't know if it would have proper adhesion directly. I have seen other people start with MDF mounting rings, then add epoxy putty for molding / large gaps; then finish with Dynatron. Just my $.02.

Kirk

Would this be the same stuff you spray on a rubber bumper before you paint? I have some of that I sprayed on some vinyl siding to change the color. Then sprayed acrylic enamel over it. It all stuck really well.
Dave