as far as I know, there has been very little success with fumaric salts of mesc. There is an unverified claim on the wiki that mesc fumarate is soluble in acetone/isa/water I think, which would mean you have to evap the polar layer to get a solid salt. There is definitely room for experimentation here, so let us know how it goes if you try fumarates out
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there is a good thread by Phlux- about cleaning resin impurities with acetone also.