Why did I get these inconsistent experimental results in silanization? I used a 50% wt mixture of Tetraethylorthosilicate + Kerosene for silanization of some glass plates with different treatment times (from 1 sec to 35 sec). However, the contact angle(CA) that I got does not seem consistent as it does not always increase as I increase the treatment time (when treatment time increases, the CA fluctuates).

What might be the reason? and whether I should change the method or the solution(s)?

P.S. I cleaned the glass with acetone in a sonicator. Then I put the glass into solution and after specific time put it into water to clean the solution from surface and then clean the remaining oil (from kerosene) by rubbing a napkin to the glass plate. Then I measured the CA.
Rubbing is a very inconsistent process, and will introduce a lot of variation. I would suggest using a volatile non-polar solvent such as acetone, hexanes, or ethyl acetate to remove residual kerosene and allowing to air dry.