Rustins Plastic Coating: Practical Advice

Brian May used Rustins Plastic Coating (RPC) to coat the Red Special. This is a two-part cold cure finish based on urea formaldehyde resins, plasticised with alkyd and reinforced with melamine. The finish is described by the manufacturer as “highly durable, heat resistant, resistant to impact, abrasion and solvents”. For MSDS (Material Safety Data Sheet) purposes it is described as an isobutylated melamine-formaldehyde resin containing isobutanol, xylene, formaldehyde (methanal), 1,2,4-trimethylbenzene, low boiling point naphtha (petroleum) and mesitylene. It is prepared by mixing one part hardener to four parts coating by volume and applied using a brush or roller.

Alternative coating choices are polyester or nitrocellulose, each of which carries their own advantages and disadvantages. Polyester is thick, resistant to wear and tear but does not mechanically or chemically adhere to the surface it is applied to, while nitrocellulose is thin but wears more readily. “Nitro” is generally favoured by guitar aficionados because it is considered to not dampen resonant sustain and will wear more readily to an aged, vintage appearance than a “poly” finish.

There are two principal issues facing builders: the first being that it is not known exactly how many coats Brian originally applied to the Red Special and the second is that difficulties can be experienced with respect to wrinkling or rippling of the finish if the correct method of application is not strictly followed.

Previous revisions of the RPC instruction leaflet stated that “a minimum of three coats is recommended but many more can be applied. Marquetry is often given as many as nine coats. When burnishing to a mirror finish at least five coats are applied“. Brian has commented that he applied “many coats” therefore this information is likely a good hint to the actual minimum and maximum required.

On www.ukworkshop.co.uk forums, several users quote personal experience or advice they received from Rustins technical support which confirms the fact that you have to either apply up to three coats within two hours to achieve the window for a chemical bond or wait three to four days for each previous coating to fully harden to achieve the window for a mechanical bond. If additional coats are applied between these windows, they will trap solvent in the hardening mixture which causes it to soften and wrinkle.

The ambient temperature affects the curing time and it is likely in U.K. conditions that many people have experienced problems because this was less than the recommended minimum of 20°C therefore previous coating(s) had not fully hardened before more were applied.

On my test block, I decided to keep it simple by applying one coat at a time and place it in our hot water tank cupboard for a few days to fully cure each individual coat before applying another. The only issue I experienced was small blemishes resulting from air bubbles becoming trapped in the hardened coating. The obvious drawback of this approach is that it will take three to four weeks to fully coat the guitar body based on three days hardening time per coating and application of seven to nine coatings.

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