I have some Purpleheart from which I intend to build gift boxes. I have looked into the topic far enough to learn that the deep rich purple color changes over time, especially after exposure to UV, and that the finish needs to be oil based.
My current thought is Minwax 40900000 Wipe-On Poly Finish Clear, as found https://smile.amazon.com/gp/product/B003W06L0S/ref=ox_sc_act_title_2?smid=ATVPDKIKX0DER&psc=1.
Unfortunately purpleheart will change to brownish over time no matter what you do. The more UV it gets the more it turns brown. I do not know of any UV inhibitors but you could see if any of the marine finishes have the UV inhibitor.
Marine spar varnish is supposed to have uv inhibitors if that helps. I had some D fir that had beautiful shades of blue and purple in it, the only piece I had ever seen like that but unfortunately it faded out and wasn't so spectacular once it did. It never saw any direct sunlight.
The use of exterior marine finishes makes sense though I'd always thought that UV blockers were to prevent deterioration of the finish itself and not the bleaching of the underlying wood.
By the way, there is a FWW article in #205 (not sure when that was, pre-2009 for sure though) about a multiyear test of exterior finishes. They found that all the consumer "exterior" finishes were basically crap - lasted no more than a year - and that even among the pro finishes only Epifanes went the distance.
Almost all lights emit some UV. So even though you do not put your project in direct sunlight it will always darken over time. Exotics like purpleheart are destined to turn brown over time. You may be able to prolong the beautiful purple/red hue but sooner or later it will turn. Even after the turn to brown it is beautiful.
Herb, the marine finishes seem to work great on the teak on my boat...and it is in the sun all four seasons. After scraping all the old stuff I do a hand sanding then teak oil and then spar finish. If you haven't tried one of the marine spar finishes, grab a test piece/put some on and leave it outdoors through all weather. Not sure how it might work on Purpleheart...
If we're still talking about the exterior teak on boats, the marine spar finishes all work quite well without additional stuff. Not sure if 303 will even layer on top of Epiphanes...
I've used Epiphanes, Varathane and Man-o-war and they keep the color of the wood until they are scratched...
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