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Shellac + Preval Question

Discussion in 'Finely Finished' started by davegardner0, Oct 8, 2018.

  1. davegardner0

    davegardner0 TDPRI Member

    18
    May 3, 2016
    NJ, USA
    I've done some french polishing on instruments and now I'm getting into aniline dyes on the surface under the shellac.

    I've used the bullseye spray can shellac with mixed results - when it works it leaves a (reasonably) smooth surface and the dye is well-sealed so I can french polish over the top without any dye smearing or coming through. However it's really hard to tell how old the spray cans are, and I've now had more than one instance where the spray shellac is clearly too old and refuses to harden.

    I mix my own flakes for french polishing, so I'm thinking a preval sprayer may be my best option. I bought one and tried it out with a 1lb cut - it sprays alright although the surface is a little bumpy. However I did see my dyes run a bit on the test piece, including a little black dye running over an inlay. I tried to keep the coats really light, but it's possible I sprayed too many in quick succession.

    So, am I missing anything here? Has anybody tried shellac in a preval? What's the recommended cut for spraying shellac in a preval unit? Any other suggestions?

    Thanks!
     
  2. dan40

    dan40 Tele-Holic Silver Supporter

    955
    Aug 19, 2015
    Richmond Va
    I recently sprayed some blonde shellac tinted with the Stewmac vintage amber dye. It started as a 1.5lb cut but I did dilute it down a bit more. I had to keep the spray nozzle back away from the piece a bit and just mist it on lightly. It took a little practice getting the technique right but overall it turned out very nice. I did finish it with a few coats of Mohawk lacquer though.
     
  3. As long as your mix is spraying OK, I don't think the problem is with your cut. The shellac isn't causing the dye to run, the alcohol the shellac is mixed with is the culprit :).

    When spraying an alcohol based finish over an alcohol soluble dye, the initial coats of finish need to be real light, more like "mist" coats instead of wet coats.

    I think you are on the right track with spraying to lock the dye in before going to the french polish (brushing or wiping at the start will most likely smear the dye), you just need to lighten up a bit on the initial coats until you've got enough shellac applied to lock the color in.

    Give it a go on some test pieces :).



    Best Regards,
    Geo.
     
    eallen and bender66 like this.
  4. davegardner0

    davegardner0 TDPRI Member

    18
    May 3, 2016
    NJ, USA
    Thanks for the thoughts! I think this is exactly my issue. The sprayed shellac dries so quickly that it's deceiving - It makes me want to spray thicker coats and also spray too many in a row. Plus the preval shoots out a lot of liquid at once. I think I was both too close to the wood last time and also lingered too long while spraying.

    How long would you suggest between mist coats? And how many would you recommend before the dye is sufficiently sealed?
     
  5. TRexF16

    TRexF16 Friend of Leo's

    Apr 4, 2011
    Tucson
    This matched pair has a base of garnet shellac sprayed with a Preval, followed up with lacquer sprayed with a normal gun. I had no complaints with the lacquer through the Preval, and like you, I used a 1 lb cut. The lighting makes it look a bit blotchy but it's actually quite even:

    Bodies side by side.png
     
    old wrench likes this.
  6. philosofriend

    philosofriend Tele-Holic

    829
    Oct 28, 2015
    Kalamazoo
    To me "mist coat" sounds like a mix with more solvent. I think that they are talking about a thin dry coat with less alcohol in the mix. Just enough spray to put some shellac on the surface without having it sit there wet for very long.

    How long to wait between coats? Ideally, until you can put your nose right up to the finish and not smell the alcohol. Don't be in a hurry with those mist coats because what you are trying to avoid is solvent penetrating down to the dye.

    The more you wait between coats the sooner the complete finish will be done shrinking. A little patience saves time in the long run.
    Experiments on scrap wood should show you what it takes to keep the dye from dissolving.
     
  7. I guess the term "mist coat" is kinda vague. Sorry for the confusion :).

    An old car painter turned me on to the technique, and he called it "mist coat". So, I've always called it the same :).

    What I meant was to use your existing 1 lb. cut and spray very light (mist! ;)) coats to start and allow enough time between coats for the shellac to completely dry. It doesn't take very long at all for light coats of good fresh shellac to dry.

    The easiest way to find out how many "mist" or very light coats you'll need to lock the dye up is to do up a test piece or two with the same materials. It should move along pretty quickly.

    I've used mist coats to lock up TransTint dyes much the same as you are, and I've also used the same misting technique to lock up decals too, although I was using different finishes with the decals - laquer on headstocks, and 2K clear on motorcycle tanks, but the technique is the same.



    Best Regards,
    Geo.
     
  8. Silverface

    Silverface Poster Extraordinaire Ad Free Member

    Age:
    66
    Mar 2, 2003
    Lawndale CA
    A "mist coat" means spraying a light fog of material *over* - not *at* - the surface from at least 12" above and letting it fall onto the piece. It' one of the few situations where the piece must be flat - not hung or mounted vertically.

    It's very difficult to do well with normal lacquers or other coatings if in aerosol cans due to lack to trigger control and poor atomization, and with aerosols I recommend misting only be used for blush reducer or flow-enhancing sprays. There are other situations, like decal "fixing", where light coats work, but misting usually results in dry spray. A *non bleeder* HVLP (one of the better types where the air is not running all the time) works great.

    With a shellac you may be able to do a mist coat over dye with a Preval, but it will be hard to control. One way or another VERY light coats need to be laid down - probably 4-5 with shellac. As @old wrench says, it's the solvent causing the dye to flow - not the shellac resin.

    Blush reducers and flow agents are different - they're not just material with a higher standard thinner content, though. They generally include retarders (slower-evaporating solvent blends) and/or surfactants (wetting agents) so the old coating can be wetted, flow evenly and dry. Additional solvent only may cause rippling (looking like large "fisheye").

    But back to the original topic - I don't recommend shellac over alcohol-reduced dye for this very reason. I only recommend shellac as a finish for restoration of instruments originally finished in shellac using French polish techniques (usually in combination with pumice as a traditional grain filler) vintage Weissenborn types and violin family instruments.

    I won't use or recommend it as a guitar finish due to lack of solvent resistance and comparatively poor durability. The pigmented versions are good sealers against tannic acid bleed, but the clears lack the stearates and calcium carbonate needed for use as a good sanding sealer.
     
  9. davegardner0

    davegardner0 TDPRI Member

    18
    May 3, 2016
    NJ, USA
    I got a chance to try the mist coat yesterday and today on a test piece and then the guitar (although not a true mist coat as described in the last post). It seems to work well! This time I sprayed from a little farther back, maybe 12" and was MUCH more careful with how much finish I was laying down. I also waited an hour or so between coats instead of 10 minutes. So far I have 4 coats on and while I don't think the dye is totally sealed yet it is well on its way. I can see absolutely no evidence of the dye moving around, and most importantly I don't see any over my inlay.

    Thanks to all who chimed in with help! In the end the 1lb cut seems fine, I was just too heavy handed with the spraying.

    I definitely know what you mean about the poor durability of shellac, and of course all of my difficulties here are due to my dye and clear coat sharing a common solvent. BUT for me the look of shellac over aniline dye is pretty great. The chatoyance that shellac shows is awesome, so I like to use it right over the dye (on bare wood) whether I'm doing an all shellac finish (french polishing over my sprayed coats) or adding some other more protective clear coat.
     
    old wrench likes this.
  10. TNO

    TNO Friend of Leo's

    Apr 25, 2003
    NC-USA
    Funny, the maple necks I've finished with shellac have held up amazingly well. In my experience it's about as durable as nc lacquer. Same with several bodies I've done.
     
  11. davegardner0

    davegardner0 TDPRI Member

    18
    May 3, 2016
    NJ, USA
    I think mechanically shellac is not THAT much weaker than lacquer. Maybe about the same hardness difference as between poly and lacquer. I think the issue with Shellac is poor resistance to solvents (alcohol especially), poor resistance to extreme humidity, and also certain people's sweat can dissolve and really mess up a shellac finish.

    That being said, I'm usually not a fan of gloss finished necks but I think a gloss shellac neck has a feeling like nothing else. It's glossy but still feels like wood somehow, plus it doesn't seem to get sticky.

    So there's pros and cons to shellac overall...
     
  12. sleazy pot pie

    sleazy pot pie Tele-Holic

    890
    May 31, 2014
    austin
    I have a guitar body and neck that I finished with shellac through a preval almost 5 years ago. While the body shows a small amount of forearm wear, the neck has held up fine.
    The forearm wear isn’t noticeable unless you are looking for it. I wouldn’t be worried about the durability of shellac unless you are totally careless with you guitar.
     
  13. TNO

    TNO Friend of Leo's

    Apr 25, 2003
    NC-USA
    The poor resistance to solvents thing is not really an issue. Plenty of humidity here in NC and have hand no issues.
     
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