Stew Mac Sanding Sealer Issue

Discussion in 'Finely Finished' started by FatBack, Oct 24, 2018.

  1. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

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    IF you are going to be using black lacquer use black lacquer primer. Mohawk makes it. Or use a gray lacquer primer. You CAN just use the black lacquer you are using for color coats - you have lacquer sanding sealer and black lacquer with chemically bond to it. DO NOT USE SHELLAC! It's sometimes used as a sanding sealer but it is not a primer OVER sanding sealer. All it will do is screw up the bond between the sealer and color coats.

    The only sanding from here on will be to fix minor runs. You don't want to surface sand between coats - it's redundant on lacquer and can introduce contaminants. And don't sand between color and clear for the same reason - you want the coats to melt into each other properly.

    Black is the most difficult single-color opaque finish to apply well, so practice on scrap until you have your application technique *nailed*. The piece needs to be hanging or attached to a paint stick so you can spray 90% of it at 90 degrees to the surface at 8-10" If it lays flat you have to shoot at an angle and the coverage will be [very inconsistent.

    Each coat should consist of 3-4 VERY light passes at a minimum, and a single coat will not cover very well. If it does you are applying too heavily, which is much worse than applying too thin. The black is applied that way; with clear you apply the first 4-5 coats like that and the last one or two can be "flood" coats to create an even, smooth, glossy finish. Again, this takes PRACTICE on scrap wood.

    When you have applied the black reasonably smooth and clear coats laid out smooth and even (where you can go straight to buffing - surface sanding is only done if the finish was applied unevenly). THEN start with the color coats on the guitar.

    Edited to add - pigmented lacquer primers are used as primers in SOME situations - as a buffer coat between a polyurethane and an enamel or lacquer, on bare (or filled but NOT sealed) wood that has knots or is known to bleed tannic acid and on difficult to coat metals, plastics and other adhesion-rejecting materials.
     
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  2. FatBack

    FatBack Tele-Meister

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    I finally got some free time and sanded the Stew Mac sanding sealer down with 320 grit and wiped clean with a cloth dampened with mineral spirits. Most likely, I've just been over sanding between sanding sealer coats.

    I probably committed "Finely Finished" heresy today by spraying the neck and body with Krylon flat white primer from Ace Hardware just now. I have used the stuff on everything and it always has worked great for me. After all this work, this bass is going to look good...even if it drives me mad.

    I am happy to finally get somewhere with my kit. I intend to block sand the white primered body and do a lot to the neck. Maybe the parts will get another primer coat afterward. Once that's all done....wait for a clear warm dry day to spray the nitro! Now that's what I've been looking forward to!

    Well....nobody can accuse me of rushing this build, lol! I guess new builders can use up some time!
     

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  3. FatBack

    FatBack Tele-Meister

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    Thanks for taking the time to post all that good info, Silverface! I am going to follow your advice about spraying when I get to the color coat and clear stage. I have a tendency to get carried away and apply coats that are too thick.
     
  4. TelZilla

    TelZilla Friend of Leo's

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    :D You forgot “always do your entire process on a piece of scrap”. Only time I've ever used the ignore function...
     
  5. Buttered Biskit

    Buttered Biskit Tele-Meister

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    Good advice but I'd add this: always let automotive primer cure for a week after you sand it flat before you recoat with an even coat of primer. IT WILL SHRINK and show scratches.
     
  6. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

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    Since this thread has been resurrected, I'll add a couple of other comments:

    1. Conventional lacquer, whether nitrocellulose or acrylic, DOES NOT CURE. It dries only by evaporation. That's where the problem with applying lacquer like paint - i.e. at the same thickness, which is FAR too much - gets people into trouble. The only lacquers that "cure" are catalyzed lacquers. But see #2 below regarding dry time....

    If a coat of lacquer is too thick it will skin over on top, trapping solvents and other volatiles underneath. It may take days to dry - or weeks, or months - or it may NEVER dry properly.

    That's why it's recommended that new and less experienced finishers apply lacquer coats in 3 VERY light passes. A single coat of pigmented lacquer should not cover completely or flow out smoothly. If it does, it's too thick! And clears won't flow smoothly ether - generally neither will until the 3rd or 4th coat.

    Lacquer is not similar to paint and the various products used in a lacquer system react with each other differently that paint primer and finish.The "solids by volume" - the stuff that actually stays on the surface - of lacquer is usually about 1/4 that of a typical enamel paint, and it takes many more coats of lacquer to build the same film thickness as paint - which it generally not necessary.

    So to repeat what was said a couple years ago, it's a very good idea to do a practice application of the whole system before you start on the real thing. Then you won't get surprised by how products work together and why it takes 10 total coats of many lacquer systems to achieve a smooth surface that goes straight to the buffer the day after the final coat is applied! Wet sanding is only needed if the surface is uneven - it's a repair method, NOT a normal part of the finishing process (except with a very few systems that take years of experience). I've worked with first-time finishers using aerosols who didn't ned to sand anything except sanding sealer and grain filler (and NEVER sand between lacquer coats! It can only make things worse unless you're removing runs or entire coats!)!

    2. Stewmac's Colortone products, like Deft "Lacquers, are "lacquer enamels". If you read the MSDS and find naphtha, mineral spirits, petroleum distillates and/or "alkyd resin" the products are a combination of lacquer and oil-based enamel - and take FAR longer to dry than conventional lacquers (Mohawk, Behlens, ReRanch, Duplicolor, VFalspar, Sherwin-Wiliams, Cardinal etc).

    A coat of properly applied conventional lacquer - whether nitrocellulose, acrylic, or a blend of the two (the resins are perfectly compatible) - dries in 30-60 minutes (assuming acceptable temperature/humidity). On the other hand, a coat of Colortone takes a minimum of 4 hours according to their product data - and based on testing, that's with a VERY thin coat. I recommend at least a day between coats.

    Their instructions also mention "cure time", which is technically wrong - it's "dry time". They also give instructions for time between coats and after final coats for sanding, which ONLY applies to their products (and, from lab testing, to most Deft lacquers as well).

    And the recommendations for "number of cans" needed are weird, as proper can agitation and material temperature, the guitar shape, spray technique, film thickness applied, and amount of overspray and "bounce" combine to determine how much material is used. You really need to start by figuring the spread rate - measure wet film thickness with a "wet film thickness gage" (easily found on Amazon), take the solids by volume (not by weight) from the product data or MSDS, calculate the resulting dry film thickness....and then cut the result by at LEAST half to account for overspray and bounce....to figure out how thick each coat will be. Then use spread rate charts from the internet, measure the size of the guitar's coatable surface in square feet.

    You'll probably come up[ with about half the amount Stewmac wants you to buy.o_O

    But those instructions are not relevant to conventional lacquers. Stewmac's entire finishing guide i meant for Colortone products. While it's a good reference for preparation and spray technique, every bit of information regarding "cure time"is irrelevant if you are using regular lacquers, sanding between coats is not normal, wet sanding, adgain, is only necessary if you sprayed unevenly and dry/cure/whatever time before final buffing is absurd.

    Even a newbie that has to do a little wet sanding before buffing can prep, seal, dye and grain fill in one day; apply another sealer coat and color, toner or clear coats the next; final clear coats on the third day, and if done in the morning buff it (preferably with 3 different cloth wheels and 3 stick buffing compounds) in the afternoon.

    Done. And if there's not grain fill (paste wood filler) an entire project with 10-12 coats of lacquer can be a 2 day project - 3 if you want to wait a day before buffing.

    IF YOU PRACTICE.

    Not wanting to practice on scrap and applying overly thick coats are very common issues, related to the #1 cause of finishing problems:

    Impatience.

    Hope that helps, especially in clarifying some critical product differences.
     
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