Waterborne Finishes Swelling MDF

      Some, but not all, waterborne finishes cause troublesome swelling in MDF surface fibers. January 2, 2012

I'm using Lorchem's HydroSand and HydroEdge waterborne on Medex. I'm having a real problem with particles swelling on the face with the first primer coat, sanded flush, swelling again with the second primer coat, and again with the first topcoat. I'm going to try wetting the raw Medex before sanding. Any other suggestions?

Forum Responses
(Finishing Forum)
From contributor R:
Can I suggest you try another type of coating?

From the original questioner:
Good suggestion, but too late on this project - it's in full panic mode.

From contributor D:
I've never seen a waterborne that works with MDF. The stuff is a complete sponge for water. I don't see how it could not swell. To me water and MDF don't mix.

From contributor A:
MLC waterborne primers and topcoats work perfectly with MDF. Not one problem in 10 years. The brand/type of finish is the problem. Not WB.

From the original questioner:
How are you prepping your MDF face? Wood?

From contributor A:
MDF faces need no prep. Wood is sanded to 150 grit. If MDF is used for raised panels or on the rare occasion we expose an MDF edge, we sand it to 240, and spot prime with a brush with BIN white shellac primer. Then sand to 320 and prime the entire panel with WB. The MLC primers do raise the grain on the MDF faces, but once sanded (one of the easiest to sand primers available) it does not grain raise again.

From contributor L:
I've used Behrs interior primer in the purple can from HD.

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