[jmimac351]

I have some dowel pin holes in MDF that I need to fill / smooth... to make them as though they were never there.  The final finish will be painted.  Any suggestions for the best product to use to do that?  I assume there is something ideal to fill the holes / sand level and smooth... something that will flex / live happily with the MDF over time - so it doesn't show up later after being painted.

I've done some searches on the interwebs... matter of fact I've seen Bondo mentioned. 

Thanks for any advice / experience.

[FullRangeMan]

I used wood powder mixed with glue, dried in 24 hours(summer).

[WGH]

Bondo. After 40 years of professional woodworking I have learned not to use anything else.
Sets in 15 minutes, paintable in 30 minutes, never needs primer

[nlitworld]

Don't use the bondo brand. That's like taking a date out to McDonald's. The problem with Bondo is it will shrink back and show the spot again, as well as dying back to stain the spot through the topcoat. Cheap product for cheap results.

 If you're talking about small dowel pins, use a two component polyester glazing putty as it will sand much smoother. My recommendations at work are always Evercoat Metal Glaze Ultra (part# 424 or 425) or U-Pol Dolphin Glaze (part# UP0714). Those mix at 2% by weight mixing ratio. Eyeballing it would be a golf ball size of glazing putty to a pea size of the hardener. Mix those up real quick and squeeze it in with a soft plastic spreader. Have everything sanded with 180grit prior to applying, then apply the putty and wait about an hour to cure, then sand flush with 180 or 220 grit. If it takes multiple applications, that's not a problem as long as it's cured and sanded before reapplying.

Give me a shout if you have more questions. I sell automotive paint for my 9-5 gig so this is easy stuff to help everyone through.

-Lloyd

[WGH]

How big are the holes? A photo would help. How are you applying the paint? High gloss show car finish or Durotex?

[jmimac351]

Thanks all.  Holes like this are what I need to fill.  Making some mods to a project.

[WGH]

Filling those holes will be a 4-step process

1. Make 9/16" long wood plugs if the MDF is 3/4" thick, the plugs can be cut off dowels. The plugs can be longer but you will have to cut them off on the inside.

2. Glue in the plugs, make sure they are at least 3/16" below the finished surface. Why? MDF expands and contracts equally in all directions with changes in humidity. A dowel is long grain and does not move at all. If the dowel was cut and sanded flush with the surface a circular shadow would eventually appear. Let dry overnight.

3. Fill the shallow hole with Bondo or polyester glazing putty. Try the polyester glazing putty. Available at Ace Hardware.

4. Sand smooth, fill and sand again, you might need 3 applications to get a smooth patch feathered into the surrounding area. MDF is soft, don't sand a trench around each hole, wrap sandpaper around a wood block.

[jmimac351]

Thank You!... Polyester Glazing Putty and a mig welder. Got it!!

You'll see how it turns out... or at least I'll know who to blame. 

[fre11111]

I recommend using saw dust and CA glue. Or even epoxy.

[WGH]

Or even epoxy.

I knew epoxy would be mentioned eventually, it always is. Sure epoxy will work but it is much harder than MDF. Sanding smooth will take longer while pressing harder. The area around the hole will also be sanded longer and harder and make it lower than the patch turning the easy fix into an unrepairable disaster. The ripples will be easily seen after painting.

I used a lot of epoxy filling mesquite (5 gallons at a time), the black tinted epoxy can be seen in the Salk speaker in my avatar. The only way to get a truly flat surface is to use a commercial wide belt sander or drum sander.

I have a buddy who would take a 4 hour job and turn it into a 6 month project by a series of cascading mistakes.

[nlitworld]

Definitely agree with WGH on gluing in plugs just shy of flush surface depth. That will be the most correct and structurally sound repair. Then flush up and sand smooth with the glazing putty and you're good to go into priming and painting steps.

Also sanding flat and smooth is easier with long sanding blocks in a criss-cross pattern to minimize digging holes into the surface.

[ketchup]

Sure epoxy will work but it is much harder than MDF. Sanding smooth will take longer while pressing harder. The area around the hole will also be sanded longer and harder and make it lower than the patch turning the easy fix into an unreparable disaster.

This is so true.  I've seen this many times filling various materials.  The harder one always ends up a little higher than the surrounding, softer material.  The opposite can also happen.  If the filler is softer, it can end up a little lower than the harder, surrounding material after sanding.   

[fre11111]

I think I should have been more clear that epoxy will only fill majority of depth in that hole and not overfill it. Putty or filler to cover the rest and make sanding an ease.

[jmimac351]

Thank you all for the feedback... this has been / is handy info, for sure. 

It has occurred to me that just buying some MDF and tracing out the form / cutting it fresh, may be the easiest / best solution.  It's not a matter of big pieces that aren't easily replicated. The MDF I have is not "heirloom quality MDF from Mount MDF in Kenya".

Thank you all!  I will share what this turns into... Should be pretty cool (I hope)