This study investigates the effects of a selection of alkaline reagents, including solutions, pastes and soap gels, used to remove an artificially aged mastic film applied over a primed canvas. Surface changes were examined using visible light and scanning electron microscopy (SEM). Ammonia, whether in solution or gelled with hydroxypropylmethylcellulose (HPMC), required a pH of 10 or higher to remove varnish at an appreciable rate, whereas ammonia in wax paste worked at pH 9. Soap gels based on deoxycholic acid (DCA) were effective at pH 9.5. The rate of removal increases significantly with the addition of benzyl alcohol, while anthracene-9-carboxylic acid (A9C) and 9-fluorenone-4-carboxylic acid (9FOC) soap gels of pH 7.9 worked quickly with out the addition of alcohol. Where reagents with a pH of more than 9 were used, some erosion of the priming was observed under SEM. Examination of the topography of residual varnish after treatment and Fourier transform infrared spectroscopy (FTIR) measurements provided evidence in support of models for the mode of action of reagents and soap gels on aged varnish. Ammonia, whether in solution, gel or paste, removes varnish by fracturing on a multi- molecular scale; soap gels not specifically tailored to remove the varnish act similarly. A9C and 9FOC gels remove the varnish by thinning from the upper surface without fracturing.