I have long harboured a desire to have an old loco quietly rotting away in a quiet corner of the railway, and now I have one; a mock up for a proposed model of one of the Tralee and Dingle Kerr Stuart 2-6-0 tanks (the second number 4) had been sitting in my attic since the early 1990s. Over the years I have robbed various parts from it for other projects. The boiler is a card tube, the rest of the bodywork is made from plasticard, and the chassis is from a modified Bachmann 4-6-0 Big Hauler battery loco (non working). This seemed a perfect candidate for a rotting loco.
The loco has been outside for about 3 months but I found its representation of decrepitude was lacking something - Rust!
I purchased some Deluxe Materials scenic rust and applied it all over the loco, having carefully read the instructions. The product consists of some fine iron particles which are mixed with a liquid adhesive to make a sticky iron paste. This is applied with a brush and once dry, the treated area is painted with a blue activator. After about 8 hours the rust is fully developed.