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Discussion Starter #1
I’m building 4 VBSS subs and am dreading the finishing process, mainly due to my lack of experience when it comes to applying paint finishes.

With the help of YouTube, I’m aware of the the many materials and methods used to seal the edges of mdf. The consensus seems to be that unless you miter mdf edges or use the “trench and fill” method to get rid of a seam, that any other method will eventually fail, meaning that over time, the mdf seam will eventually reappear - even visible through a layer of veneer.

I understand the popularity of thick paint coatings like Duratex. I even ordered some (sprayable variety) to apply to 2 of my 4 subs. The other 2 subs I want to coat with a semi-gloss finish, but want to solve the mdf seam problem if possible before doing so.

In fabricating the trim pieces for my subs, I did miter them where possible, but applying a roundover on them will likely mean having to deal with the usual mdf edge issues.

I’d like to stick with a water-based paint for the final finish. At the moment, I’m considering applying a vinyl sealer over the mdf, based on YouTube research that indicates vinyl sealer is more effective than other commonly used methods in sealing mdf.

It’s also supposed to level flatter than other sealers and thus requires less sanding. It can also be used under a water-based paint.

Does anyone have experience or insights on using vinyl sealer over other mdf edge solutions?

Thanks!
 

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FTR rattle can Kilz for sealing MDF, as cab prep continues
larger areas / starting out- cup gun spray or light roller coats
let dry thoroughly and repeat /prep / sand/
as needed
when the seams and your OCD in agreement,
then you can start to finish
 
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I have never heard of vinyl sealer, but it sound interesting and I think you should go for it and report how it works back to the rest of us! The MDF line sure is annoying, especially that it can (and does) show up even under veneer. My fix for this has become to use thick veneer panels vs. paperbacked normal veneer, but it's not always practical.
 

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Yeah. That seam. I never read about the "trench and fill" but I get the jist of it (I imagine) and it seems plausible.

I'm working on some MDF cabinets right now and decided to try a combo of things I hadn't tried before. I spackled the seams using "DAP Alex Plus spackling". I like this stuff. Seems to stay on no matter how shallow the flaw I'm trying to cover up and it's easy to sand.

Over that I'm priming with Zinnser BIN Shellac Based Primer. This stuff dries fast and sands well as well.

I've just finished sanding my first coat of primer and went back over all the boxes with some touch up of the spackling. For the most part, my seams are invisible. I touched up where they weren't. I think I'll be happy with this.

I'm not trying to dissuade you from going with the vinyl. Just throwing out a finish method. I'll finish with an acrylic. You can use the spackle underneath your vinyl I'd imagine to fill in the seams and flaws.
 

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I'm surprised no one from the OEM speaker manufacturing industry has shared their trade secrets on here for eliminating the seams in MDF. They don't bother me. The labor involved in trenching is enough of a deterrent for me.
 
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Discussion Starter #6
Since my original post I went down to Woodcraft and bought a rattle can of Mohawk vinyl primer. I set up an mdf test piece which I prepped using 220, 320 and 600 grit sandpaper. I used the 600 grit on the basis of several anecdotal accounts that 600 grit sanding actually deposits fine material into and helps seal mdf pores.

I couldn’t locate my mask, so when I started spraying, I made sure I was upwind as there was a slight breeze blowing. Even being upwind, the smell was instantly overwhelming, confirming what I had heard about how noxious vinyl sealant is.

So before I have another go at it I’m off to harbor freight to buy a charcoal mask. I can’t image spraying this stuff in my basement without using some serious venting.
 

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Discussion Starter #9
The Harbor freight mask works! I was able to apply 3 more coats of vinyl sealer on my mdf test piece:

C2F4B62D-AE29-4078-BE07-81F8148CB37F.jpeg
478CE535-E6B4-499E-BB5C-19724C5E4DA7.jpeg


You can still see some pores. I will add a couple of more coats to see if it makes a difference. One I’m satisfied, I’ll apply several coats of cabinet paint.
 

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I use Zin Binnser Shellac primer. That stuff is the best. Easy to apply and it seals MDF perfectly. Buy in a quart can and use a roller to apply and you are done in a day. You can apply multiple coats as it dries real fast, about an hour.

The tricky part is sanding if you are going to apply a top coat. After painting the primer, sand down the box and make sure any high spots do not show any MDF when the box is smooth. If you sand through the primer, reapply.

When spraying paint on the primer, you need to sand between coats to make sure you do not have any primer showing after sanding. I prefer using lacquer over enamel (you can find Rustoleum lacquer spray in the automotive department but colors are limited). If you like satin finishes, look at Rustoleum oiled rub bronze color. Looks fabulous and easy to apply. Satin has an advantage over gloss. You don't really have to worry about orange peel or an uneven surface as the satin finish hides the imperfections that gloss surfaces reveal. If you are going to use a clear coat over the gloss coat, sanding is even more critical as it shows an obvious defect if you sand through the clear coat and reach the top coat.

Advice. make sure you fill in all seams and sand it until it is smooth. Seams will show up no matter what finish you use as a fine indented line unless it is filled porperly.

I have made about seven subs so far, ported sealed, passive radiators, dual drivers, you name it. If you are using the sub for music, sealed is the way to go. The bass is tight and with the proper QTS, will sound great. But sealed subs do not go that low unless you equalize it. The best home theater sub I made was a 15" RSS390HF-4 Dayton sub in a vented box and a 500-watt amp. Sure, the box was huge (IIRC, it's a 26" cube) but it played the intro bass on Live, Die Repeat with no problems (I guess it helps that vented drivers roll off quickly after their F3 point).

If you like bass, you gotta go with 15" drivers. You can actually feel the bass. 12" drivers can't do that as it cannot move much air unless it has a super long excursion (e.g. Sunfire subs).
 
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The drywall mud trick works really well, I was skeptical of the hardness until I tried it. I used the setting type as it’s harder and more solid, don’t use lightweight or easy sand mud. You just make a mix just thin enough to brush on and then rub in by hand just to the edges, and leave just enough on to sand a bit. Since it’s rubbed into the end cuts, it just fills and it’s easy and sands easily. It’s a one and done solution, just prime and go.
 

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The drywall mud trick works really well, I was skeptical of the hardness until I tried it. I used the setting type as it’s harder and more solid, don’t use lightweight or easy sand mud. You just make a mix just thin enough to brush on and then rub in by hand just to the edges, and leave just enough on to sand a bit. Since it’s rubbed into the end cuts, it just fills and it’s easy and sands easily. It’s a one and done solution, just prime and go.
Funny, I've been thinking today about using the mud method, especially since my Benjamin Moore cabinet paint peeled off my vinyl sealed test piece. What brand did you use?
 

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So I guess I did use the easy sand bag after rattling on but all the setting stuff is harder than any of the tub stuff. The buckets of lightweight and the dust control stuff sets about as hard as smearing butter on the wall. I think I used 20 minute, it’s nice because it will be dry enough to sand that day if I recall. Just the usg bags at the depot

https://www.usg.com/content/dam/USG...-joint-compounds-easy-sand-submittal-J621.pdf
 

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So I guess I did use the easy sand bag after rattling on but all the setting stuff is harder than any of the tub stuff. The buckets of lightweight and the dust control stuff sets about as hard as smearing butter on the wall. I think I used 20 minute, it’s nice because it will be dry enough to sand that day if I recall. Just the usg bags at the depot

https://www.usg.com/content/dam/USG...-joint-compounds-easy-sand-submittal-J621.pdf
Perfect. I have some left over from the bathroom finish.
 

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Yeah. That seam. I never read about the "trench and fill" but I get the jist of it (I imagine) and it seems plausible.

I'm working on some MDF cabinets right now and decided to try a combo of things I hadn't tried before. I spackled the seams using "DAP Alex Plus spackling". I like this stuff. Seems to stay on no matter how shallow the flaw I'm trying to cover up and it's easy to sand.

Over that I'm priming with Zinnser BIN Shellac Based Primer. This stuff dries fast and sands well as well.

I've just finished sanding my first coat of primer and went back over all the boxes with some touch up of the spackling. For the most part, my seams are invisible. I touched up where they weren't. I think I'll be happy with this.

I'm not trying to dissuade you from going with the vinyl. Just throwing out a finish method. I'll finish with an acrylic. You can use the spackle underneath your vinyl I'd imagine to fill in the seams and flaws.
Good afternoon. I just read that you used Alex Plus Filler. This is NOT compatible with Duratex. Duratex will crack and lift when applied over this filler. We recommend using Elmer's Carpenter's Color Change Wood Filler. This has been used for years without any issues.
 

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Good afternoon. I just read that you used Alex Plus Filler. This is NOT compatible with Duratex. Duratex will crack and lift when applied over this filler. We recommend using Elmer's Carpenter's Color Change Wood Filler. This has been used for years without any issues.
Thank you for adding that information! It's much appreciated.

I'm priming with Zinnser BIN shellac-based primer and I'll be painting with PPG Satin "Breakthrough" acrylic. I should be ok with this but it's definitely good to know about this since my next project will most likely be finished with Duratex. (I'm tired of trying to hide MDF seams with priming and sanding :) )
 
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