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Old 22nd August 2011, 04:12 PM   #321
GM is offline GM  United States
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FWIW, the primers I used were waterproof, though not sure about today's EPA mandated formulations, but you're right, they shouldn't be wet sanded per my comments WRT surface area. Since you spelled it out, I saw no reason to repeat it, but as you now have pointed out, the best reason is that if you break through it the MDF may not be protected enough to keep from either immediately swelling, or worse, blistering at some later date after the project is finished.

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Old 26th August 2011, 12:05 AM   #322
Renron is offline Renron  United States
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Waterproof primer. Well I learned sumptin' new again today!
Thanks,
Ron
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Old 14th September 2011, 01:15 AM   #323
Billyo is offline Billyo  Australia
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Hi all,

I'm just applying the last coats of an automotive acryric clear from a spraycan to my cabinets. The instructions on the spraycan obviously refer to application to a car body, but it basically skips a wet'n'dry sanding step and says to apply a polish (after a few days drying time) to start getting a glossy finish.

Just wondering whether these instructions would also apply when being applied to well-primed MDF? Would the end result be the same (albeit take longer).

I would love to avoid having to slosh water around during a wet'n'dry sanding - as last time water got into the MDF substrate, which subsequently caused swelling, which then required complete repreparation of an entire panel - wasting well over a week.

I'm itching to finish and get some noise out of these babies.
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Old 2nd October 2011, 10:21 PM   #324
Billyo is offline Billyo  Australia
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Polish did nothing observable.

But happy to report that going straight from a spray finish to using fairly aggressive cutting compound formed the beginnings of a gloss and skips the step of using wet-n-dry.

It does take a fair bit of elbow grease however - I bet that using wet sandpaper would be a lot faster.

Took a good 30 minutes rubbing to form a gloss on the top of 1 speaker that roughly matched the sort of glossy PVC (?) covering that is currently popular in entertainment furniture.
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Old 2nd October 2011, 10:38 PM   #325
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Quote:
Originally Posted by Billyo View Post
Polish did nothing observable.

But happy to report that going straight from a spray finish to using fairly aggressive cutting compound formed the beginnings of a gloss and skips the step of using wet-n-dry.

It does take a fair bit of elbow grease however - I bet that using wet sandpaper would be a lot faster.

Took a good 30 minutes rubbing to form a gloss on the top of 1 speaker that roughly matched the sort of glossy PVC (?) covering that is currently popular in entertainment furniture.
Cutting compound won't take out the peel, though.
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Old 3rd October 2011, 02:57 PM   #326
Renron is offline Renron  United States
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Sandpaper will though.
Start at 1500 LIGHTLY and see if that cuts the orange peel . Obviously use a sanding block, and stay away from the edges.
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Old 4th October 2011, 12:04 AM   #327
Billyo is offline Billyo  Australia
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I'll keep rubbing and see what happens.

Here's an interesting observation:

The sides of the cabinet are joined to the top and bottom using PVA white glue and a butt joint - the join is distinctly although unobtrusively visible, even when the joint was routed and sanded so smooth that it was impossible to detect the joint using touch.

The baffle, however, was glued to the rest of the cabinet using that foaming, expanding Polyurathane glue, the overflow was then routed flush and then sanded - that join is virtually invisible through the paint... its *just* detectable if I look really hard.

Hopefully it will stay that way.
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Old 4th October 2011, 09:55 AM   #328
Billyo is offline Billyo  Australia
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Although not up to the standard that would be required for a gloss black finish, I'm quite satisfied with the outcome of the cutting compound - especially since the paint was just applied with spraycans from Supacheap auto.

There is some orange peel if I look really carefully, but it tends to be well hidden in the general glare of the white, which is very forgiving. I'll keep rubbing and see if I can eliminate it all and then switch to a finer polish. Its kinda difficult to photograph, but I hope the pic below gives an idea of what it looks like:

Click the image to open in full size.
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Old 4th October 2011, 02:28 PM   #329
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Quote:
Originally Posted by Billyo View Post
The sides of the cabinet are joined to the top and bottom using PVA white glue and a butt joint - the join is distinctly although unobtrusively visible, even when the joint was routed and sanded so smooth that it was impossible to detect the joint using touch.

The baffle, however, was glued to the rest of the cabinet using that foaming, expanding Polyurathane glue, the overflow was then routed flush and then sanded - that join is virtually invisible through the paint... its *just* detectable if I look really hard.
Two different material impedance mismatches and what I see in the photo implies you're either using your hands or a grinder/buffer and applying too much pressure. Note too that PVA shrinks [hope yours was done before painting], so not a good 'filler' unless allowed/heat forced to shrink and one or more coats used. Better to use the appropriate glazing or spot putty to seal seams, blemishes: Putty - Body Filler, Glaze and Related - Products

As Renron implied, best to always use a sanding block for any flat areas when doing any cutting, including rubbing compound, if there's no professional buffing machine available. On 'soft' surfaces and/or where there's a hardness mismatch such as at a glue or filler/MDF, wood or similar joint, using other means tends to provide a variable pressure, cutting the softer of the two more just that tiny bit more, and why 'perfect' looking/feeling joint seams and/or 'feathering' of filler often don't look so perfect once finished to a high luster.

Also, seeing/feeling any imperfections on a dull surface is tricky at best for most folks without lots of experience and why I previously suggested the two tone primer application to be able to easily see them/do minor seam fill. Most of the cycles, cars I shot were foreign jobs: MG, Triumph, etc. and the body work was so uneven enough that it wasn't uncommon to have to do at least several thick coats of two tone primer to even them out enough to shoot dark colors to even approach American auto finish standards.

Forgiving indeed! Except for pearl, I prefer mirror finish bright white over other colors since as long as it sits in either shadow free or full shadow lighting, it will be too hard to look at for folks to notice any minor flaws.

Anyway, at this point though, good luck with it and hope you don't burn through the clear since it can be tough to clean off all the cutting oil residue enough to allow a re-coat and why block dry sanding is preferred plus adding one or more extra block sanded coats to get a thick enough ~uniform surface to minimize labor/material loss during the final finish procedure.

GM
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Old 4th October 2011, 04:10 PM   #330
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I think I may have to spend a day and read this whole section. I have done a piano Finnish, It's a bit of work but it's well worth it.
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