Thin polyester gelcoat with styrene, not acetone, at about 5% to 10% by volume. Styrene lowers viscosity so the gelcoat atomizes and flows out smoothly while keeping the resin chemistry intact, whereas acetone just flashes off and can leave the film weak and porous. Add the styrene before the MEKP, and stay at or below 10% so you do not weaken the cured coat.