Method of preventing polymer scale deposition

A method of preventing polymer scale deposition in a polymerization vessel during polymerization of a monomer having an ethylenic double bond and having the general formula (I):

• CH.sub.2 .dbd.CXY (I)
• wherein X is a hydrogen atom or the methyl group;
• Y is a hydrogen atom, an alkyl group or a group represented by the formula:
• --COOH, --COOM where M is an alkali metal or an ammonium ion, --COOR, --OCOR, --OR where in the formulas R is an alkyl group, --CH, --C.sub.6 H.sub.5, C.sub.6 H.sub.4 Z where Z is an hydrogen atom, --OH, --CH.sub.3 or --CH.dbd.CH.sub.2, or --CH.dbd.CH.sub.2, wherein said polymerization is carried out in a polymerization vessel of which the inner wall has been previously coated with a first coating liquid comprising:
• (A) a water-soluble anionic dye, and
• (B) a salt of a metal selected from the group consisting of alkaline metals, zinc family metals, aluminum family metals, tin family metals, iron family metals, chromium group metals, manganese family metals, copper family metals, silver and platinum family metals, to form a first coating,
• wherein the first coating has been dried and then coated with a second coating liquid comprising:
• (C) an alkaline earth metal compound, and at least one member selected from the group consisting of:
• (D) a water-soluble anionic dye and
• (E) an inorganic colloid, to form a second coating which second coating has also been dried.