Low quality MDF

[PoorboyMike]

I'm building a 5.1 set up for a friends mom and the baffles are chamfered all the way around and will be painted. The problem is, I got some crappy MDF from Home Depot and the end grain is almost fuzzy feeling after routing the chamfers. I've put a coat of sanding sealer and a coat of varnish on them and they still are fuzzy feeling. I honestly think I could use a whole gallon of varnish on these and it would soak it all up like a sponge. :M

Any ideas on how to seal these up so I can actually paint them? I already have a lot of time into these and don't want to start over. Would smearing PVA glue along the chamfer work?

[ahaik]

I'm building a 5.1 set up for a friends mom and the baffles are chamfered all the way around and will be painted. The problem is, I got some crappy MDF from Home Depot and the end grain is almost fuzzy feeling after routing the chamfers. I've put a coat of sanding sealer and a coat of varnish on them and they still are fuzzy feeling. I honestly think I could use a whole gallon of varnish on these and it would soak it all up like a sponge. :M

Any ideas on how to seal these up so I can actually paint them? I already have a lot of time into these and don't want to start over. Would smearing PVA glue along the chamfer work?

Maybe Bondo ?

[cjd]

slightly thinned spackle works wonders on MDF edges.

C

[Chris7]

Old shellac (stuff that's too old to use for anything else) works wonders for stiffening up MDF. Not sure I would use it here since you've already used varnish.

PVA diluted with water will also work. Don't smear on undiluted PVA directly otherwise it can make sanding difficult and uneven.

[engr_dave]

and will be painted.

Apply a generous coat of high-solids primer-surfacer... wait a few days and sand. It will fill MDF edges perfectly.





If you're doing a clear finish, you can use a high-solids lacquer sanding sealer, but it may take a few coats to completely fill the edges.

[Hdale85]

Wow how long did it take you to get that glossy finish? Did you use a sprayer?

[technimac]

Wow how long did it take you to get that glossy finish? Did you use a sprayer?

My guess is that he probably used the one in the first photo :W

[Hdale85]

Yeah my bad I overlooked it Still wondering how long it took to get a finish like that though.

[PMazz]

I use thinned wood putty.

[engr_dave]

Yup, I spray almost everything. I have a couple of production guns and some touch-up guns. Old fashioned, not too expensive models from Home Depot. I haven't gone the HVLP route yet.

This post https://www.htguide.com/forum/showpo...0&postcount=13 from my build thread on those subwoofers covers some details on the finish. The "secrets", if there are any, are

1. Perform sanding steps with a large, unbacked (or thinly backed) block. It helps keep the surfaces flat. MDF is, of course, flat to begin with but the process of trimming edges during build can result in divots and "wavyness". I have a variety of power sanders, but I most often sand with a full sheet of sandpaper wrapped around 7" x 11" slab of 1" MDF. My large hands grip this comfortably.

2. High-solids automotive primer-surfacer. Great stuff. Fills well, dries very quickly, sands easily into a fine powder.

3. Inspect the primed surface carefully before final finish. Fill imperfections with spot putty and sand or do another round of primer-surfacer. Final sanding with 320 makes imperfections easy to see in reflected light as the surface is almost "polished".

4. Give the surfaces plenty of time to dry and shrink before final finish. A couple of days is good, a week is better.

5. In warm temperatures, lacquer can be difficult to spray without getting a dry, pebbled finish, especially on larger surfaces. May need to go to a "slow" thinner and/or retarder... or wait until shop is cold.

6. Apply plenty of finish - three full wet coats minimum. You DON'T want to cut through later when polishing. 8O On the other hand, be careful to not overdo it on non-horizontal surfaces which will result in runs & sags.

7. Color sanding takes some practice to completely smooth the surface without cutting through the finish. Pause to clean and inspect surface often. As soon as 90% smooth, STOP. Polishing compound will take care of the rest.

In the end, a hand-polished finish takes more time than applying and finishing veneer, but not much more. If you have access to a spray booth or dust-free work area you can get almost the same results with modern two-part acrylic enamels and basecoat/clearcost systems... without the color sanding and polishing.

[engr_dave]

Sorry Dougie, last post was a long-winded way of NOT answering your question.

how long it took to get a finish like that

Finishing process takes about a week, with most of the time waiting for drying. I probably have about 20 man-hours in the finish of those subwoofers, with color sanding and polishing taking at least 8 of those (10 surfaces and 24 rounded edges just takes time).

Also in the above post, steps 3 and 4 should be reversed. Let primer dry and shrink before sanding.

Have fun!

[capslock]

In my experience, fillers (water and solvent based) do not penetrate well into the MDF and make a mess while sanding. I use two or three coats of slightly thinned 2K PU clear lacquer. It was originally intended for wooden floors, fills well and becomes extremely hard and will not clog the sanding discs. I have also used this as a clear coat on top of a color coat, but this particular lot seems to be too old and develops tiny bubbles, so I just use it to seal MDF.

[PoorboyMike]

Thanks for all the replies guys.

I've worked with a lot of MDF in the last few years and this is the worst stuff I've ever used. My NatP's have painted baffles and all that was required was a coat of primer. I didn't even bother trying to prime these edges they were so soft. It actually has a texture similar to rough suede leather. It will be the last time I get MDF at Home Depot. From now on it's the lumber yard, even though it's 40 miles away.

Last night I brushed some PVA on the edges and it really seems to have hardened up good. I'm not making hardly anything on these so I didn't want to go buy more stuff to fix them up. I'll take a shot at painting them later today if the sanding goes well.

[capslock]

PVA will clog your sanding disk.

[impala454]

Apply a generous coat of high-solids primer-surfacer... wait a few days and sand. It will fill MDF edges perfectly.





If you're doing a clear finish, you can use a high-solids lacquer sanding sealer, but it may take a few coats to completely fill the edges.

I know this is a pretty old thread, but found it in a search, what exactly is "high solids primer surfacer"? I just made a trip by both home depot and lowes and didn't see anything with that description... where would I find the stuff?

[Edge540]

you will find it at an automotive paint store. i believe O'Reilys Auto Parts carries Nason products.

[impala454]

cool thanks

[Paul Ebert]

I've had the same experience with MDF at Home Depot. What they sell at Lowes seems to be much better. I only buy it from HD if I want a small (2' by 4') piece (which Lowes doesn't sell).

[djg]

Not fine woodworking but...

This is a triska sub I built for my computer sound system, from PE showcase. This is Menards MDF finished with one can of Rustoleum stone look spray texture paint. This paint fills Mdf ends very well. No prep or primer, just one rattle can.

[tech9_79]

I really think it all depends what the big box stores have at the time, I picked up some MDF from homedepot and lowes, the stuff homedpot had was much denser and darker color almost red, lowes stuff was lighter and softer. Next week it may be a different story.

anway here or some great threads on sealing/expansion and new ways of doing a high gloss finish using 1k waterbased urethanes, pretty cool stuff

sealing/expansion

make sure to read to page 4 and 5
highgloss urethanes

[impala454]

i guess it's just really tough to finish without having to veneer

[JonP]

I really think it all depends what the big box stores have at the time, I picked up some MDF from homedepot and lowes, the stuff homedpot had was much denser and darker color almost red, lowes stuff was lighter and softer. Next week it may be a different story.

Yeah, I too have seen the quality vary a lot. Good thing to do, is find some serious lumber yard, contractors supply type of place, they should have the good stuff.

I (as always) would put in a vote for the good ol' shellac based sealers, like Zinnser's Seal Coat. Alcohol based, soaks in deep, dries fast, makes a hard surface that sands nice and smooth. Compatible with just about anything. I think they also have an added solids shellac product, they also have some other non-shellac based primer/sealer. Haven't used either of those.