I have a DIY Bass VI (from USACG) - which means either buying Creamery or Novak pickups (I would love to) - or (being cheap) buying a Jaguar set second hand and buying an individual pickup .. which I did 2 x MIJ and a SD quarter-pounder for the bridge.
I bought a new pickup cover for the SD and following some very sagely advice made a cup of coffee and dunked the pickup covers in.
Observation:
The WD Music pickup cover has stained to a respectable yellow.
The MIJ pickup covers must be made from American tooth enamel and are as white as before.
I've ordered more WD covers and I hope the colouration has more to do with absorption properties of the plastic than strength of coffee.
Comments
The MIJ/CIJ plastic is firm, inflexible and possesses an unbearable whiteness of being.
The tinted plastic used for “aged” covers usually looks a bit fake.
Remove the covers. Use some very fine abrasive if you like to de-gloss them (i use 2000 grit, but you can start coarser and go to 2000 if you choose). This will also soften the corners slightly which looks more natural. Next daub on the stuff and you will immediately think you've ruined the covers! Leave it to dry for about 5mins then rub it off with a soft cloth.
If the initial effect is too subtle then simply repeat.
If the colour is too strong/uneven use a bit of white spirit on a pad/cloth to soften the effect. I found that if you are thorough then white spirit seems to remove 98‰ of it. If you want a really subtle effect apply the stuff, remove with white spirit, apply again, remove with white spirit etc. After a few cycles it does seem to slowly build up a little colour, and the white spirit loaded pad (which by now is dark brown) can also be used to apply a subtle coat.
Once you've experimented a bit you can use a sponge, or a pad and can create areas which are deliberately more aged, and it will tend to stay within scratches and nicks if you want to go beyond ageing to relicing.
I don't have a picture, but I aged a new Dimarzio pickup cover which was naturally a slightly different colour to the Fender pickup covers it needed to complement. I also added some wear and damage to fit the relic vibe of the instrument it was going on. @CountryDave now has that guitar and might be able to provide a picture by way of an example.
http://www.thefretboard.co.uk/discussion/89942/caspercaster#latest
it looks a little darker than the other two pickups, but is bang on as that’s where my picking hand sits.
I played an old Squier for about 25 years, gigging a lot. Over the years the pickups yellowed and discoloured and the one that went darkest was the bridge pickup - just where this one has.
https://www.thefretboard.co.uk/discussion/36718/fake-dirt-and-ageing-plastics-a-bit-of-metal
And another example halfway down this page
https://www.thefretboard.co.uk/discussion/108084/les-paul-prototype-build/p10
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