Best methods for finishing MDF

[davepellegrene]

Re: Best methods for finishing MDF

I had a real mind blowing idea after reading through this thread. :D

What about rubbing on the wood glue (possibly diluted), then lay thin fiberglass cloth over the joints and add a little more glue on top of the fiberglass?

I would imagine it could be made very smooth if you pulled a squeegy over the area.

I need to order some fiberglass cloth and give it a try.

What do you guys think?

I would think you would have to do a trench so you don't get a build up on the seam. It would take a lot of work to feather it out other wise.
So far the glue method seams to have put down a nice base to build off of.
Dave

[Erich H]

Re: Best methods for finishing MDF

I would think you would have to do a trench so you don't get a build up on the seam. It would take a lot of work to feather it out other wise.
So far the glue method seams to have put down a nice base to build off of.
Dave

I was thinking about the really thin nicely woven cloth. Isn't that easier to work with? Something like this 1.4 oz cloth:

http://cgi.ebay.com/FIBERGLASS-CLOTH-1-4oz-x-50-Light-Weight-Fiberglass_W0QQitemZ370218611983QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item5632bfb50f#ht_500wt_900

I would think you could put it down extremely thin. And using wood glue versus resin, it shouldn't only save time, but you could also skip the resin chemicals as well. If by some chance you had to feather it out, maybe covering the entire side would keep that from happening. The price is cheap enough and you wouldn't need to cover the front or back.

I will order some and give it a try.


With the cold weather, it's a great time to try these experiments because you can put the test pieces outside in dry cold air, then bring them inside to a warm room. Doing that quite a few times should speed up any cracks that would eventually show up.

My guess is that the epoxy covered joint will do the best job. I just used some of that on my last subwoofer build and it's rock solid. If it was able to soak into the mdf prior to curing, those corners should be like one solid piece......hopefully! :D

[davepellegrene]

Re: Best methods for finishing MDF

Got a chance to sand the pieces today and put some primer on them. I didn't put primer on the sanding sealer or the bin 123. I think I will apply more of the same product until it is ready for paint. I don't want to jump to any conclusions, but so far I like the Titebond II for sealing the MDF. Especially on the end grain.
As far as the seam goes I like the "Icing" polyester finishing putty. It seems to be about the same hardness as the MDF so feathering it into the MDF is working really nice. It also dries really smooth. The Epoxy seams to have worked well in the trench but it doesn't feather well to the MDF. The fiberglass resin sealed the MDF I would say better then the wood glue but its a lot more work and toxic to work with.
I may throw in another sample with a trench then seal it with the Titebone and then fill the trench with the Icing to see how that will work.
Dave

[davepellegrene]

Re: Best methods for finishing MDF

Got some results. I ended up priming all the samples with Automotive lacquer primer from a can just so the results would be the same. This test was really about hiding the seam not sealing the MDF.
After seeing the results of how each product sealed the MDF I would say the fiberglass resin came in first, sanding sealer 2nd, Tightbond II 3rd and Bin 123 shellac last.
As far as filling the end seam goes I would put the fiberglass resin first, Tightbond II second, Bin 123 3rd and the sanding sealer last.
All samples were sanded after applying them with 150. Then coated with primer and sanded with 220. Then coated with primer again then sanded with 600. I then applied Rust o Leum High Performance Enamel from a spray can. Just because I had it. I did a tack coat then two more coats about 10 minutes apart. I had the samples under an exhaust fan and when I turned it on got some dirt in the samples. Opps!
Here are the pics with paint.

1 Epoxy in trench and skimmed with Icing because it left a pretty deep indent. You can see the feathered edges from the Epoxy through the primer. It may do better with a sealer before priming.


2 Body putty in trench with fiberglass resin coating


3 Body putty in trench with no coating


4 Titebond II

[davepellegrene]

Re: Best methods for finishing MDF

5 Bin 123 white pigmented shellac


6 Fiberglass resin coating


7 Benjamin Moore Sanding Sealer


8 Polyester Finishing Putty "Icing", in trench, with no coating.

[davepellegrene]

Re: Best methods for finishing MDF

I think next I will do some more test pieces combining the processes to get the simplest method for sealing and preventing the seam from showing. Also putting on a nicer top coat.
So far all the trench methods and one (fiberglass coating) have hidden the seam. If I don't see a change in any of the samples over the next few weeks I may take them up to the ledge in my shower to introduce them to some moisture.:D I should probably use my daughters shower though. She spends a lot of time in there.
At my link below I have seperate pics for each test, so you can see the process as it goes on. Just scroll to the bottom of the page.
Dave

[lunchmoney]

Re: Best methods for finishing MDF

Nice work dave!

Please post results again in a year... that's the real test :p

The last speakers I did (the Poor Man's) have a couple of coats of 75/25 elmers wood glue/water (I don't recommend, too thick to spread well... then 4 coats of heavy build primer... then 4 coats of chromacolor vinyl paint.

They had zero seams showing about 2 weeks after I painted them... in about 6 months I expect seams to start showing.

[davepellegrene]

Re: Best methods for finishing MDF

Nice work dave!

Please post results again in a year... that's the real test :p

The last speakers I did (the Poor Man's) have a couple of coats of 75/25 elmers wood glue/water (I don't recommend, too thick to spread well... then 4 coats of heavy build primer... then 4 coats of chromacolor vinyl paint.

They had zero seams showing about 2 weeks after I painted them... in about 6 months I expect seams to start showing.

This was my first attempt with the wood glue. I can see were thinning and having it soak in would be smoother. The think I didn't like about the wood glue is that it left a rougher finish. I had it sag in one spot and wasn't easy to sand out. I am going to try the trench method then a couple of coats of thinned wood glue to seal it up then the icing to fill the trench. This would be my method of choice if it works good.
The fiberglass resin ended up being the simplest process without a trench. If it holds up in the seam. But down side is if it sags coating the whole speaker
it will be more work to smooth out.
Like you say time will tell.
Dave

[Erich H]

Re: Best methods for finishing MDF

I recall reading that Bondo auto body filler is simply polyester resin and talc powder mixed together until it's very thick.

I bet you could buy plain polyester resin from HomeDepot and mix in enough talc until you get the liquid consistency you need for a nice smooth coat.

Epoxy resin at the consistency they use for fiberglassing should soak into the mdf and self level. Pour a little on and then use a squeegy to "push" it into the seam. It should make for a rock soild and level piece to prime.


Your testing does look good.

[davepellegrene]

Re: Best methods for finishing MDF

Unfortunately I already have a few updates. Today I checked the samples and had some surprises.
Epoxy in the indent. Looks like finish cracked some more around the sides were it feathers into the MDF.


Titebond II also cracked really bad over entire peice.


Fiberglass resin is already starting to show the seam. If you look close to the top left light reflection you can see it.


I have three samples left that are not showing the seam yet. The one with polyester finishing putty "Icing" in the trench, The one with auto body putty in the trench and the one with auto body putty in the trench with fiberglass resin coating.
Dave

[jimlg]

Re: Best methods for finishing MDF

What are the possibilities of speeding up the time affects by moving the test pieces inside and outside, or maybe spend a night in the fridge. I realize the conditions would be a little extreme, but it would be a sort of stress test??

[davepellegrene]

Re: Best methods for finishing MDF

What are the possibilities of speeding up the time affects by moving the test pieces inside and outside, or maybe spend a night in the fridge. I realize the conditions would be a little extreme, but it would be a sort of stress test??

I was planning on putting them on a ledge in my shower up by the ceiling. That should introduce them to a lot of moisture twice a day. I was going to wait until the paint cures completely. Moisture seems to be MDF's worse enemy. It expands a lot when exposed to it.
I plan on making up quit a few more samples trying different finishes and combination's of treatments. So I am really not in to big of a hurry. Once and if I come up with some that work well I will try all kinds of tests to stress them.
You are the second one to suggest the hot cold method so I will try it.
Thanks Dave

[bobbarkto]

Re: Best methods for finishing MDF

Dave,

Wood, wood products and almost every finish imagineable have very low thermal coefficients.
Typically in the range of less than .005% at a temperature range +90F to -20F.

Relative humidity is the thing to watch. Dimensional change is orders of magnitude greater for just a few percentage point change in RH.

If you want to see some paint films crack or craze, then freeze the samples.
Usually only artist type emulsions and antique finishes will crack or craze at these low temperatures. Below -20 it's another game.

I was planning on putting them on a ledge in my shower up by the ceiling. That should introduce them to a lot of moisture twice a day. I was going to wait until the paint cures completely. Moisture seems to be MDF's worse enemy. It expands a lot when exposed to it.
I plan on making up quit a few more samples trying different finishes and combination's of treatments. So I am really not in to big of a hurry. Once and if I come up with some that work well I will try all kinds of tests to stress them.
You are the second one to suggest the hot cold method so I will try it.
Thanks Dave

[jonpike]

Re: Best methods for finishing MDF

I think next I will do some more test pieces combining the processes to get the simplest method for sealing and preventing the seam from showing. Also putting on a nicer top coat.
Dave

Very interesting... great to see actual experiments!

I was wondering though, if I could vote for adding another to the list, which would be thinned Zinnser Seal Coat. (not the BIN pigment filled stuff) Typical usage for this kind of thing, is cut the Seal Coat 50:50 with denatured Alcohol, so you have a rather thin liquid that will soak deep into the MDF. 2-3 separate coats, as much as it will soak up each time.

I'm thinking this might work differently, in that being so thin it should soak deeper than most of these, and thereby make a thicker stabilized/sealed layer in the MDF. I've always been able to put an impressive amount into the MDF I'm working with.

I've never used the BIN stuff, and don't know its relative thickness or penetrating ability, though.

[davepellegrene]

Re: Best methods for finishing MDF

Very interesting... great to see actual experiments!

I was wondering though, if I could vote for adding another to the list, which would be thinned Zinnser Seal Coat. (not the BIN pigment filled stuff) Typical usage for this kind of thing, is cut the Seal Coat 50:50 with denatured Alcohol, so you have a rather thin liquid that will soak deep into the MDF. 2-3 separate coats, as much as it will soak up each time.

I'm thinking this might work differently, in that being so thin it should soak deeper than most of these, and thereby make a thicker stabilized/sealed layer in the MDF. I've always been able to put an impressive amount into the MDF I'm working with.


I've never used the BIN stuff, and don't know its relative thickness or penetrating ability, though.

Bin is made by Zinnser it could be thinned to do the same thing. I believe the only real difference is that it is pigmented for covering with paint. I could be wrong on that.
I will add that to the list. The sanding sealer I used seem to do a decent job sealing the wood. I don't think any product is going to stop the seam from showing with out using the trench though.
Dave