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Painted MDF shelves with water based primer. How do I fix the raised grain?

I made a huge mistake and painted a LOT of shelves with a water based primer...2 Coates. The grain swelled and now the surface is bumpy and grainy. Can I fill the spaces or sand it? Help!!!
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Posted 2014-01-19T05:41:04+0000  by Ldavis916 Ldavis916




At this point, you should sand the surface the best you can with a 150 grit sandpaper and then re-prime with an oil based primer. After the primer dries, re-sand it again with 220 grit sandpaper. After dusting, paint with either an acrylic or oil paint. Oil paint will give you a much harder, less gummy feeling surface upon which to place items.


Depending on how good you want the finished product to look , it might be less work to just "bite the bullet" and start ove again with new MDF.  :(


Hope this has helped.

Posted 2014-01-19T06:43:09+0000  by ordjen
It's a pantry and my father spent a chunk of time cutting and routing the edges. It's not horrible but it's not my intended look. I will try your suggestion. :(
Posted 2014-01-19T06:57:16+0000  by Ldavis916

Hello Ldavis916!


Old school woodcrafters intentionally used water-based products as their first coat to deliberately swell the wood grain.


Once dry, they sanded using increasing number (finer) sandpaper to create an exceptionally smooth surface before completing their topcoats.


Often this progression took several days and the finished surface was glass-like.


Even though it was not your intent, you've followed the finishing path that produces outstanding results.



When selecting sandpaper, make certain to look for the "Non-Gumming" paper (typically the backing is blue).


This paper easily releases sanding debris, preventing the paper from quickly building a layer that coats the sanding surface and prevents contact with your wood.


Your Hardware Associate can show you the difference in the yellow backed and blue backed sandpaper.

Posted 2014-01-21T14:42:00+0000  by Pat_HD_ATL

Wetting the wood to swell and open the grain, was done mainly when a deep, dark stain was desired and the closed nature of the grain wouldn't allow a penetrating stain to penetrate. If merely a super smooth finish is desired, use of a sanding sealer is the easier route.


Sanding sealers are kind of like a varnish, but are extremely sandable, allowing the roughness raised by the sealer to be sanded away. One warning, old fashioned sterate sealers are not compatible with urethane varnishes. Urethanes have dedicated sanding sealers, or may be used with one of the "universal" sanding sealers, such as that by Minwax. Curiously, the Minwax Universal Sanding Sealer is water based, but still results in a very smooth surface.


Where the final surface is to be painted, a pigmented oil primer serves the purpose of setting the grain, just like a sanding sealer, or not lifting the "grain" of MDF. It leaves a smooth, water impermiable surface upon which either acrylic or oil based paints may be used.

Posted 2014-01-21T17:28:27+0000  by ordjen

You should have no trouble keeping those shelves your Dad made Ldavis916!


Whether you use Parks Sanding Sealer, Varathane Sanding Sealer, or MinWax Sanding Sealer, they are all water-based products designed to make the grain stand up ... in preparation for sanding down to the smoothest surface possible.


Parks Sanding Sealer.jpg  Varathane Sanding Sealer.jpg  MinWax Sanding Sealer.jpg


The old school woodcrafters used these water-based products to improve their results and you can too!


WOW! I really love generational projects ... Dad makes it great; son preserves it for the next generation.


Once done, come back and show The Community the handiwork of both you and your Dad!

Posted 2014-01-23T16:19:35+0000  by Pat_HD_ATL


In the case of wood which has already been puckered up with a water based primer, a high build, pigmented  oil primer or oil enamel undecoater, will be of more benefit that a clear sanding sealer. Without pigment, they simply don't have enough film build up to mask the uneven surface.


I have in the past used oil enamel undercoaters to mask the poor paint jobs which customers had inherited in their new to them homes. Enamel undercoater would go a long way toward masking the "ropiness" left by the last "painter"who used  either poor brush technique and/or poor quality latex paint. A heavy coat of enamel undercoater followed by a thorough sanding,  and a finish coat of lower sheen enamel went a long way to bring back the woodwork.

Posted 2014-01-24T04:41:51+0000  by ordjen
Absolutely Ldavis916,

Sanding is easier!

Sand the raised wood grain with 220-grit and you'll find that the next coat of sanding sealer will completely seal the surface.

Don't use filler!

The ridges will only be turned into more visible anthills on the surface of your MDF.

Not attractive or functional at all.

Posted 2015-12-08T21:15:58+0000  by Pat_HD_ATL
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