Re: Damar/Gum Turp/Boiled linseed compatibility
Frankly, thats a dreadful choice for a final varnish. Its going to go a lovely shade of yellow over the next 15 to 20 years (or sooner) and prove to be almost impossible to remove.
Damar won't dissolve properly in mineral spirits. Don't use oil-varnishes as a final varnish. (Damar in turpentine - no oil! - would be a slightly better choice, but I'd suggest that Damar as a final varnish should be regarded as obsolete).
Hardware store boiled linseed oil is wholly unsuitable to use in oil painting.
I'd encourage you to use one of the modern synthetic varnishes designed not to yellow, and to be easily removable, such as Gamvar, or W&N gloss artist's varnish.