Filling MDF Edges

Tips on filling and sealing exposed MDF "grain" so finishes won't soak in. October 14, 2006

I have tried Clawlock, 2K polyester fillers, etc, and experienced the usual problems of dry time and soak-in with the Clawlock, versus the manual application of polyester automotive glaze filler. They are great systems, but I was still trying to find the ultimate one application sprayable filler that would dry to sand to powder within minutes, and I finally found it.

I picked up a two part urethane casting resin from a company called RenCast. It has two components - 100% solids casting resin with microspheres for great sanding, and the cured material actually foams a little to expand like other moisture cure urethanes. The problem is that it is a bit too hot – it will set up within five minutes, less in hot weather. I took the chance and thinned it to spray viscosity with acetone (about 15%), loaded it into the spray gun, and sprayed a bunch of milled MDF and MDF edges, then immediately cleaned the gun. The leftover thinned material still set up in ten minutes.

It coated in super uniformity due to spraying, but had great hold-out, with little soak-in. It cured to sand to powder within about ten minutes, topcoated immediately. If you want instant fillers for a rush job, with great mil thickness and lightning speed dry time - this is the trick. You’ve got to work fast, unless you buy a slower casting resin.

Forum Responses
(Finishing Forum)
From contributor A:
How about veneer taping the edge?

From the original questioner:
Good idea, but I make wide beveled edge bases for art/sculpture in 1-1/4" MDF. Edgebanding is clearly the way to go on flat, straight edges for sure.

From contributor B:

I believe that the original questioner was referring to the profile of raised panel doors and filling them in preparation to paint. Edge banding is not an alternative. We, too have difficulty with the preparation of MDF door panels for paint. Your idea sounds like it works exactly as what we have been trying to achieve. The pot life scares me.

From the original questioner:
Pot life scared me out of trying this too, but I had a rush sample to get out that required color-changing metallic painted onto these beautiful sculpted MDF panels by Interlam (Art Diffusion series - surface milled by CNC to look like water), and not wanting to wait for hourly dry time of conventional fillers like Clawlock, this idea pushed me past the scary pot life issue.

In 85° F temperature, the acetone still slowed the pot life down to a workable time, and I was able to pour the unused material out, which was hitting exotherm, and then solidified in a paper cup five minutes after I emptied the gun. This is not a super practical trick, as Clawlock is still the way to go on large jobs, but if you like to live on the edge - it was a rather cool experience. Definitely useful in short runs though.

From contributor C:
Wood glue thinned 50% with water seems to work well. Apply with a brush, then sand.

From the original questioner:
That certainly makes sense - I don't know why I never tried it. Shrinkage, soak-in, dry time, and sandability might not be quite as good as the 2K resin, but it sure sounds like a great sealer and every bit as productive.

From contributor D:
I've been using drywall filler for milled edges. It's a little messy and the dry time can be a bit slow but it sands and takes paint beautifully. I tried it first on some cabinets I built for my house and the paint still looks great after almost two years.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor P:
Has anyone tried using CA glue- "super glue"- to seal the milled areas of MDF? If you use the thin viscosity glue it soaks in and can be activated with accelerator. This gives you a rock hard surface that can either be sanded or re-milled to a mirror finish. It only takes a few minutes to apply.