Check the surface level in all directions before pouring material. When the pattern, model, form or part is in the casting position and about to be poured, verify it's secured so it wont move or float during the pour. If you are making a single section , visually inspect for any backdrafts that would prevent pattern removal. The pattern or model material, whether it's wood, plaster or of another porous composition, must be moisture proof. If not, the porosity will harbor moisture that can combine with the casting and flaw the mold surface.
Pores can also bond the pattern and plastic together which makes demolding nearly impossible.
It dries in minutes, seals porosity and builds quickly to a thick surface. As each coat dries, lightly sand with very fine sandpaper. The final coat becomes extremely slick with extra fine steel wool. As mentioned before, avoid alcohol-reduced sealers, namely shellac.