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MDF Edge Preperation
MDF provides an excellent surface for paint. MDF's one weakness is the porous cut edges. I've tried a lot of things in the past. Lacquer undercoater and tight grained wood edgebanding work reasonably well. Lacquer undercoater, while much better than quick dry oil based primer, leaves a rough edge that needs to be sanded. The process needs to be done twice to get an edge free of pores. Sanding and keeping the narrow surface flat and square isn't easy. I've never tried the often suggested glue size treatment, but I think it would similar to, and more time consuming than, primer. Edgebanding is less labor intensive, but the seam will telegraph sooner or later due to the ongoing expansion and contraction of dissimilar materials.
I've found that lightweight glass micro-balloon based spackle is faster and better. While you can apply it with a spackle knife, its messy and slow. Custom applicators/scrapers are the only way this method will be faster. Applicators/scrapers are best made with thin stainless steel bent on a sheetmetal break. My preferred brand is Red Devils OneTime, Dap's Fast 'N Final is a bit thicker/drier and not as easy to use. The trick is to spread it so it gets pushed into the pores and then scrape it off as fast as you can. You want the surface to remain as clear as possible, i.e. no visible spackle. When it dries, a light sanding (220) is all that's needed. When you sand the edge you'll notice that the dust is white. While it doesn't look like it, the spackle fills the pores and seals the material. The spackled edge will take the primer just like the face.
Woodweb discussions on MDF prep:
Prepping MDF for Paint
Priming MDF Edges for Finishing
Sealing the Edges of MDF
The later has a comment that says "Don't make the mistake of building up a 1-1/2" MDF edge with two layers of 3/4" material and then lacquering it. The glue line will witness.". The only time I've had a glue line show is when the glue line was too thick. The only time that happened was when I used a wide (4-6"?) buildup strip. My standard buildup strip is less than 2" wide. Using a toothed glue spreader and plenty of spring clamps will make witnessing a non issue... I now use a fine tooth spreader for all glue-ups, including veneer.
I've found that lightweight glass micro-balloon based spackle is faster and better. While you can apply it with a spackle knife, its messy and slow. Custom applicators/scrapers are the only way this method will be faster. Applicators/scrapers are best made with thin stainless steel bent on a sheetmetal break. My preferred brand is Red Devils OneTime, Dap's Fast 'N Final is a bit thicker/drier and not as easy to use. The trick is to spread it so it gets pushed into the pores and then scrape it off as fast as you can. You want the surface to remain as clear as possible, i.e. no visible spackle. When it dries, a light sanding (220) is all that's needed. When you sand the edge you'll notice that the dust is white. While it doesn't look like it, the spackle fills the pores and seals the material. The spackled edge will take the primer just like the face.Woodweb discussions on MDF prep:
Prepping MDF for Paint
Priming MDF Edges for Finishing
Sealing the Edges of MDF
The later has a comment that says "Don't make the mistake of building up a 1-1/2" MDF edge with two layers of 3/4" material and then lacquering it. The glue line will witness.". The only time I've had a glue line show is when the glue line was too thick. The only time that happened was when I used a wide (4-6"?) buildup strip. My standard buildup strip is less than 2" wide. Using a toothed glue spreader and plenty of spring clamps will make witnessing a non issue... I now use a fine tooth spreader for all glue-ups, including veneer.
