Question about Re-Spraying Laquer after curing

[GUNSNGUITARS]

Hi
I just got done letting my guitar body cure for about 2 weeks.  I wet sanded and because I'm new at it, I had a couple of sand through's. I've heard nightmare stories about re-spraying laquer over a cured surface.  Is this True??I also never had the level surface because I didn't sand enough between 4 coat intervals. I wet sanded to about 1200 grit so I have a smooth finish. My plans are to spray about 3 more thinned coats and let it cure again.  Anyone forsee a problem with this??

 

[jackthehack]

What did you sand through? Is there dye/toner or other finish under the gloss top coats that got sanded through? 4 coats of sprayed lacquer is nowhere near thick enough to wet sand without issues as you have discovered, I generally do at least 10-12; it's OK to spot sand any drips/runs in between coats, but that needs to be done by hand.

If you just sanded through the top gloss coat and there is no issue with underlying finish, first shoot another 8-10 coats of lacquer and let that cure before you start re-leveling. I would not thin the lacquer, while that may speed the drying time you still decrease the thickness of each coat and just wind up spraying more coats so the net time savings are near 0.

What grit are you starting with to wet sand?

 

[GUNSNGUITARS]

I actually sprayed 10 coats. I sanded after 4 coats and 8 coats. It was basically a very light 400 grit sand to level the surface.  I wet sanded after 10 coats and waiting 2 weeks, and yes I did sand through the  dye into the quilted maple, not real bad, but bad enough where you can see there is no more laquer there.  I think my main problem was I didn't spray near the edges thick enough. I began wet sanding with 600 grit because I was having a hard time getting all the "dimples" out, or orange peel I think you guys call it. I really don't think I level sanded between coats enough, or just didn't spray enough. Some guys say they don't level sand until your flow coats, other guys say sand after 4 coats before spraying your next 4.  Someone else at a guitar shop just told me to rough it up lightly with 400 grit before re-spraying again to prevent crawling. Here is the link to my work. The finishing part of this is new to me. Thanks for your response!!
http://www.thebackhomeboys.com/pics/guitar/TimsNewGuitar/index.html

 

[jackthehack]

There are probably as many techniques as there are people spraying lacquer, and I've done my fair share of sanding everything back bare and starting over...

I generally never sand back until I have all my coats done, with the exception of sanding back any drips/runs between coats. One thing that is kinda hard to develop some consistancy on is keeping the overall applications of coats as uniform in thickness as possible.

 

[Tonar8352]

Tim,
Your guitar build looks great so far. I have some ideas for you but I’m guessing until I get a little more info.  What brand of lacquer and how much did you thin it?  Also is it 680, 550, or 275 VOC and what thinners did you use if any at all? Lastly what equipment did you spray it with? 

 

[jackthehack]

Tonar - I got a DeWalt 200 PSI / 5.2 cfm@90 PSI compressor to run my nail guns/impact tools; what spray gun would you recommend to use with this rig? Going to graduate from spray cans, will primarily be shooting gloss top coats with it.

 

[-CB-]

Even those drips and runs in lacquer - look bad when ya do 'em, but when they dry and shrink, are hardly a big issue, and ... are so easy to sand back later.

 

[Tonar8352]

Jack,
I would suggest you go to you local automotive paint supplier and tell them how much cfm your compressor is running and ask them to recommend a gravity feed HVLP gun. Make sure they know that you will spray thin materials so they give you the correct fluid needle, fluid nozzle, and air cap.
 
I use an Iwata LPH-400 that is rated for 9.5 cfm.    It gets be a  hungry with 5.2 cfm but I did use it for a long time with a compressor rated at that.  I just had to wait for it to catch up and it worked fine.

 

[jackthehack]

Ouch! That Iwata unit's a tad expensive; what advantages does it have over other gravity feed HVLP guns in the $150 range? What size needle/nozzle do you use to spray lacquer? Waiting for it to catch up isn't an issue as I don't exactly have an assembly line going. Yet....

 

[Tonar8352]

Ouch! That Iwata unit's a tad expensive; what advantages does it have over other gravity feed HVLP guns in the $150 range?

Probably not much other than the stupid answer that I like to buy the best.  That is why I suggest you go see the automotive paint guys.  They have to get great paint jobs and they have the stuff to do it. I just went in and asked for the best on the market and got it.  I’m sure they can hook you up with something that will work great for you.

 

[jackthehack]

Gotcha; if I continue to build about a guitar a month I can cost justify spending the $400,  just wondered if it had some special edge in terms of performance or available options over the less expensive ones. One certainly can't argue with your results!

 

[GUNSNGUITARS]

Tonar

I used Behlen Insturment Grade Laquer. I didn't thin it until the final 2 coats.  I am using a spray gun from Grizzly.com.  H7671 comes with two guns. I am using the one with the 1.4mm tip. I sprayed at 30 psi.  When I wet sanded it  down, some of the "dimples" or orange peel were so deep that I wound up sanding through in a couple small spots, all on the edge of the guitar near the binding.  I'm guessing I didn't spray thick enough on the edges. The Air compressor is a 4HP Coleman I believe, and I think it is rated at 9.1cfm. Not sure what you mean by VOC. Right now I have it in pretty good shape except for the sand throughs. I've just heard horror stories about people spraying over laquer after it has cured, only to have it crack like spider webs later.  It has cured for about 3 weeks now. I just ordered another coat of laquer and it should be here today and I plan on spraying at least 2 more coats to fix it.  Thanks for advice guys.

 

[jackthehack]

VOC (Volatile Organic Compound) is a rating that simply put denotes the ratio of actual lacquer resins/pigments to the solvents expressed in weight ratios/%s that evaporate out they are suspended in to apply; some states/countries like California/Canada have restrictions on what can be sold due to air quality concerns from the evaporative solvents that are released into the atmosphere as the applied lacquer dries. Here's a link:

http://www.maclac.com/Miscl/VOCguide2k.pdf

By law the VOC rating should be printed on the container.

I'm interested in seeing what Tonar's recommendation(s) are as to preferred VOC ratings for lacquer, I live somewhere without any restrictions.

 

[Tonar8352]

There are several California Air Quality Management Districts (AQMD) that regulate the amount of VOC in any given paint coating including lacquers, oil base, and even latex.   It gets really wacky and would take four pages to explain so lets get to the real world.

If I can get my hands on 680 I’ll use if in a heartbeat.   Many of you across the country should still be able to get 680.  

I use a fair amount of 550 since it is still readily available to me.  After I finishing adding thinners to get it to act right it is no longer VOC compliant anyway.  Shame on me!!!!!!

If it gets to the place where we are forced to use 275 I will no longer finish guitars unless the manufactures come up with some new magic to make the stuff work better.  

 

[Tonar8352]

I've just heard horror stories about people spraying over laquer after it has cured, only to have it crack like spider webs later

You should be able to spray over that lacquer with no problems.

Finish checking is caused by several things.
1. The use of too much sanding sealer before the finish causes checking.  Imagine a mirror lying on a bed mattress; the mattress will not support the rigid glass so the glass will break with any kind of surface tension.  
2. Using too much finish material.    Since you have sanded it back far enough to burn through I don’t think that will be a problem.
3. Extreme temperature changes.
4. Lacquer gets old and brittle and starts to check.

When I wet sanded it  down, some of the "dimples" or orange peel were so deep that I wound up sanding through in a couple small spots, all on the edge of the guitar near the binding.

If your orange peel was that heavy you did not get good flow and level with the finish.  I suggest you try thinning your finish with some lacquer retarder.  I blend my thinners using 2/3 parts lacquer thinner and 1/3 part retarder.  You have to experiment with this because every manufactures thinners are different.  The retarder I use  is Val-Spar and it is pretty strong so it slows the dry time way down and I get better flow and level.  It also extends my recoat times to an hour or hour and thirty minutes.  I also tend to spray all of my coats real thin and just do more of them.  


I keep sand paper away from the edges during the whole finish process because the edges do not build finish well.  I do run a really wore out 3M Sanding sponge over them one time as I’m level sanding to dull rounded edges.   When I wet sand I use  a really really really old sanding sponge with Murphy’s Oil soap in Purified Water to dull the finish. .  I stay off all sharp edges until I get to the buffer.  

I still get burn throughs once in awhile but the more I do it the less I get.

 

[GUNSNGUITARS]

Tonar and Jack
Thanks for the advice.  I will try to thin the laquer to see if that gives a more Flat surface.  I believe that the Behlen Instrument Grade already has a retarder in it, but I will check.  Everyone says Mc Fadden Laquer is much better.  I may try that on my next project. Thanks again guys
tim