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Greg
Polishing pitch could cost $60 or more per kilo to land these days.
Gum Resin or Rosin can certainly be used for laps so long as you add some fine wood flour which will give it properties more similar to traditional pitch .
A 'tropical' lap mixture was sold by Astro Optical for many years to mirror makers - it was supposed to work better for our warmer conditions . I certainly used variants of it to make hundreds of mirrors when I was there .Eric Witcombe told me he thought the pitch in the mixture was just to give the lap a familiar black colour and probably makes no difference .
My old copy of the AOS publication Building a Low Cost telescope lists the mixture for an 8" mirror at 20 dgerees C as :
1lb Gum Rosin
3 oz of wood flour
55ml Castor Oil
1 oz pitch
Adjust Ratios of Rosin to oil for hardening and softening .
Mixing the wood flour into the rosin brings back many old memories
I don't know where you will get wood flour these days but perhaps a wood working shop where they do finish sanding ?
You would need a very soft lap for figuring F4 mirrors .
Hope this helps
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