Hoax Riddle

A letter:

My dear Gaynor—So you would like to know the answer to that riddle? Don’t be in a hurry to tell it to Amy and Frances: triumph over them for a while!

My first lends its aid when you plunge into trade.

Gain. Who would go into trade if there were no gain in it?

My second in jollifications—

Or. The French for gold. Your jollifications would be very limited if you had no money.

My whole, laid on thinnish, imparts a neat finish
To pictorial representations.

Gaynor. Because she will be an ornament to the Shakespeare Charades—only she must be ‘laid on thinnish’, that is, there mustn’t be too much of her.
Yours affectionately,
C L DODGSON.

John Fisher, in The Magic of Lewis Carroll, calls this a ‘hoax riddle’ (Gaynor is Gaynor Simpson, ‘one of Carroll’s young Guildford friends’, and this letter dates from 1874). Carroll later compounded the gag:

My dear Gaynor—forgive me for having sent you a sham answer to begin with.
My first—Sea. It carries the ships of the merchants.
My second—Weed. That is, a cigar, an article much used in jollifications
My whole—Seaweed. Take a newly painted oil-picture, lay it on its back on the floo, and spread over it, ‘thinnish’, some wet seaweed. You will find you have ‘finished’ that picture.
Yours affectionately,
C L DODGSON.

Lots of possible answers, including: res (‘thing’, necessary for trade) + ‘in’ (where the party is held): resin. ‘Co’ (as in limited company) + ‘pal’ (a friend for jollity): copal. But I’d like to think this has a photographic answer. One that occurs to me: ‘silver’, which certainly helps trade, and ‘nitrate’ (the ‘night rate’ for parties). Another might be: ‘Co’ (= limited company) and ‘odeon’ (the location of enjoyment): Collodion. We’re getting tenuous, though.