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I’ve made an outdoor business sign for a pub in my town. I used a variety
of exotic hardwoods including Morado (or Bolivian Rosewood) cut thin
(3/16″) onlayed onto a basswood substrate to draw the pub logo I also painted parts of the basswood. I knew that this sign would face harsh elements ranging from cold winters in Montreal as well as winds and UV rays so I chose an exterior varnish to protect the sign. The product I chose was Minwax Helmsman exterior urethane varnish glossy in a spray can. I sprayed 2 full cans on the sign spraying all of it wet on wet and waiting 20 minutes between coats, I consulted the company before doing this and they recommended spraying it wet on wet, I put on 5 thin coats. The varnish dried everywhere on the sign except it would not dry on the Morado. I left it for 3 weeks and it stayed tacky. After consulting with Minwax they informed me that the product was not tested on Morado, they recommended wiping the surface with a rag slightly dampened with mineral spirits to remove the tacky surface varnish. I tried this and the varnish
started stripping off in blotches so I scraped all the varnish off the
Morado leaving the rest of the sign intact. I varnished again this time using liquid varnish applied with a brush, I put on one thin coat diluted with 10% mineral spirits and it remained tacky after ten days. I called Minwax again and they suggested I sand the Morado to a lower grit, I originally sanded it to 400 so I scraped the varnish off the Morado again and sanded it to 180 and put on a thin washcoat of the varnish diluted with 20% mineral spirits and after 10 days it’s still tacky. What can I do to varnish this Morado? I’m trying to avoid having to remove the Morado pieces and gluing on another dark colored wood since that would make this job a big money loser. The customer is getting impatient and I don’t know what else to do. This is the first time I’m working with Morado and I didn’t know it was this difficult to varnish. I was considering placing another type of finish which dries more easily on wood (such as shellac) as a first coat and brushing two coats of Minwax varnish on top of this but I don’t know if they are compatible or if the shellac will dry on the Morado. I also thought of wiping the surface of the wood with acetone to remove some of the resin from the wood to allow the varnish to adhere better. Does anyone have experience working with Bolivian Rosewood? Please advise what can be done to make the wood take the varnish.
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Replies
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Shellac might be your best bet as a barrier. All finishes will adhere to shellac successfully. Use a super blonde de-waxed shellac. Try it on some scrap first though. Having said that, I am a little surprised that the rosewood has caused you as much trouble as it has. It's been a while, maybe 5 years since I finished some of the same timber, which admittedly was done with pre-catalysed lacquer, and I had no problems after spraying a preliminary overly thinned coat of the lacquer. In the past I've finished very oily woods like teak and Bombay rosewood with varnishes, but usually the first coat or two was 50/50 varnish and mineral (white) spirits. This advice is given tentatively, but do try the shellac plus varnish trick on a spare piece first to be sure this works before turning to your actual piece. Slainte, RJ.
Must have double clicked and posted the same message twice, hence the deleted message.
*Rosewoods and all Dalbergia species contain an oil that's chock full of phenol compounds, which act as antioxidants, meaning they inhibit oxygen. All oil based products depend upon taking in oxygen from the air to cure. In effect, the oil in the wood prevents the varnish from curing. You have to remove the varnish, period. I'm sorry, but it's a hard lesson learned. The dewaxed shellac will act as barrier and is a perfect solution. Jeff Jewitt
*Here's a UK contribution.My understanding is that french polish is ideally suited for finishing rosewood.Two-pack curing systems ( non oxidizing) I believe are used in guitar finishing and chess set makers often talk of french polish sealers followed by varnish finishes etc . Rosewood surfaces can be difficult and though oxidizing varnishes may well dry, their adherence may not be satisfactory (as per the 'chemical' explanation offered by Jeff Jewitt i.e. presence of oils and phenols) due to inhibition at the interface and the fact that the wood is impermeable again because of the oils and phenols. Yes it would dry by oxidation but maybe it would not stand any abrasion. Musical instrument makers also use this wood. I get the impression some people are allergic to the wood which again would suggest it is not finished to an extent that the surface is completely sealed off.From Dr Fredk G Page at Kington Herefordshire UK assisted by John Martin (ex Croda Resins Ltd).
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