MDF Subwoofer box, Need primer or something?

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Forum topic by chetrog posted 04-20-2015 03:02 AM 2133 views 0 times favorited 11 replies Add to Favorites Watch
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84 posts in 1461 days

04-20-2015 03:02 AM

I am going to be building a subwoofer box, but I have never painted one before. I have always used carpet, but I want to paint the box “MDF”. I am testing how spray primer is using, and it isn’t working good. The edges of the box has a very noticeable differences. After sanding the joints and using couple coats of primer and spray paint, you can still see the boxes end grain. Any suggestions on what type of primer I should be using. Maybe use primer in a a gallon jug etc. ? I assume I don’t want to use water based primer on MDF. I ever tried using Joint Compound but without using a ton of it, i could still see the difference. Any suggestions?

I just set some scrap pieces of mdf and took a picture of where im seeing the line that I will be trying to hide after i primer etc then paint.

-- I had a stroke a few years back, and sometimes the words dont come out as well as I would like.

11 replies so far

View bigblockyeti's profile


5116 posts in 1718 days

#1 posted 04-20-2015 03:32 AM

MDF is a heck of a sponge. You might be able to use an epoxy based primer to better seal it, especially the edges. I’ve brushed on a slow set epoxy cut with lacquer thinner to a latex paint like consistency. It seals better and builds quicker than polyurethane.

View timbertailor's profile


1594 posts in 1421 days

#2 posted 04-20-2015 03:46 AM

MDF is a heck of a sponge. You might be able to use an epoxy based primer to better seal it, especially the edges. I ve brushed on a slow set epoxy cut with lacquer thinner to a latex paint like consistency. It seals better and builds quicker than polyurethane.

- bigblockyeti


Even a latex paint is better than nothing.

-- Brad, Texas,

View Airframer's profile


3043 posts in 1950 days

#3 posted 04-20-2015 04:02 AM

You can seal the edges with a couple skim coats of wood glue before paint.

-- Eric - "I'm getting proficient with these hand jobbers. - BigRedKnothead"

View bobtimms's profile


5 posts in 1938 days

#4 posted 04-20-2015 04:11 AM

Sheetrock mud works well also but must be sanded (easy) when dry. I mud mdf edges. sand when dry then primer. Primer I use is BIN.

View pjones46's profile


1001 posts in 2640 days

#5 posted 04-20-2015 04:22 AM

Surface faces come from the factory sanded to 150 grit or better, essentially paint-ready. Scuff-sand the surface quickly with 120-grit or 150-grit sandpaper to remove any dirt and grime and to provide for better adhesion of the primer coat.

Always use a solvent-based primer

Solvent-based primers (oil-, alcohol-, or lacquer-based) are a must, something like Bin Primer Sealer wooks well also. Never use a water-based product for the initial finish coat. The wood fibers will swell too much when they absorb the water, and you’ll get what is, in effect, raised grain on the surface that will not sand out. After the surface has been sealed with something else, though, a water-based paint will not affect the MDF adversely. I have used latex paint over properly sealed wall paneling and trim molding, but for painted furniture or cabinets, I prefer the finish quality of oil- or lacquer-based paint that is applied with a spray gun. You can also use oil based paint thinned with Flood Penetrol with a short nap roller where the Penetrol helps the paint to flow out and yeild a nice finish. You may need more than one coat.

The only real difficulty that arises when painting MDF is what to do about the edges, which are more porous than the surface—similar to the end grain of lumber—and drink in most of the finish. I’ve known woodworkers who go to the trouble of edge-banding the MDF. That approach takes more time than the method I prefer, and, no matter how well the edge-banding has been applied and trimmed, a seam still may show at the very edge.

I use drywall compound to fill the edges, whether they are cut squarely or shaped with a router bit, and I apply the compound liberally with a finger or with the palm of my hand. Unlike spackle or conventional wood putties, drywall compound has a soupy texture, so it’s a little sloppy going on. But after it dries, it sands off easily.
For edges shaped with a router, you can use the same router bit as a scraper to remove the excess globs of compound before touching up the edges with 220-grit silicon-carbide sandpaper.

-- Respectfully, Paul

View MrRon's profile


4764 posts in 3241 days

#6 posted 04-22-2015 06:14 PM

This might be a job for Bondo?

View Goss's profile


9 posts in 2238 days

#7 posted 04-22-2015 10:17 PM

If your edges are clean, no saw marks or dents, all you need to do is paint latex wall paint on the edges, leave to dry, sand down and repeat. It bonds the fibres, fills the gaps and sands easily to a good smooth finish ready for the application of primer. I honestly think you would be mad to do your edges any other way.

-- cut twice, measure once, or is that the other way round?

View bobasaurus's profile (online now)


3447 posts in 3181 days

#8 posted 04-22-2015 10:47 PM

Shellac might make a good MDF sealer before final painting/finishing. Never tried it, though.

-- Allen, Colorado (Instagram @bobasaurus_woodworking)

View Logan Windram's profile

Logan Windram

346 posts in 2459 days

#9 posted 04-23-2015 12:13 AM

Sherwin Williams Vinyl Sealer. I think it has to be sprayed, but perhaps not.

View bonesbr549's profile


1548 posts in 3064 days

#10 posted 04-23-2015 12:38 AM

I made many a mdf projects for the HS band, and a lot of things will work. Shellac will be great seals anything and it will stick to anything and anything will stick to it (top coat). I’ve used shellac, Kilz(shellac basically). To be honest one of the best and cheapest, is drywall compound. It goes on easy and and you can work it (sand) to final form easy. Alternative is auto body filler. Harder, but more expensive and a tad harder to work.

I’ve used most but mostly what ever I happened to have at the time since I was donating it.

Have a good one, cheers!

-- Sooner or later Liberals run out of other people's money.

View NewfieDan's profile


50 posts in 2646 days

#11 posted 04-24-2015 09:19 PM

My son is into electronics. One of his buddies owns a stereo sales and installation business. Where will the sub woofer be used? In a vehicle or at home?

If, in a vehicle they most usually get covered in a carpet to match the vehicle interior.

As for painting, for in a house the latex idea from Goss sounds like the simplest solution.

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