It is not impossible to find examples of "preciser" being used in published texts. From Google Books:
However, such examples can be considered negligible compared to the use of "more precise":
The usual guideline given for selecting comparative forms is to use -er for monosyllabic adjectives and some disyllabic adjectives, particularly ones that end in unstressed syllables: precise does not meet this criterion. But there are no particularly definite rules, so I'm not really sure how anyone could argue that some particular form is "valid" or "invalid". That is to say, I don't understand what these words would mean in this context. I think it would be preciser to speak of preferences and customary usage. (My own preference is for "more precise": you can see that I tried out "preciser" in the previous sentence, but I don't like the way it sounds there.)
See the answers to "Conundrum: 'cleverer' or 'more clever', 'simpler' or 'more simple' etc".