I have heard about the use of an artists' paint called gouache to create rust effects, both from magazine articles and when attending clinics at NMRA events. However, my earlier attempts were not successful.
Having constructed a couple of tanks that are part of my Cheers! Craft Brewers industry, which is located in a rather old building, I wanted some rust effects on the tanks so they are in keeping with the ramshackle look of the building. The tanks are made from white PVC pipe, capped with styrene and with styrene seams. The tanks were painted with a gloss spray paint which I then coated with a matte clear coat because the gloss was "too glossy".
Gouache is a water-based artists paint (sometimes called poster paint). It has one huge advantage over oils and acrylics when it comes to weathering - once dry, it can be completely removed simply by getting it wet. For a weathering project this is a nice feature because it is easy to start over if you are not happy with your first attempt.
I purchased a set of gouache paint tubes from Walmart Canada for under C$20.00.
I first applied dots of burnt sienna gouache randomly on the tanks, placing most of the paint under the seams, as follows:
The gouache needs to dry completely, which only takes a few minutes.
I then held a slightly damp tissue on a paint dot or group of dots for a few seconds to soften the paint and then pulled the softened paint downwards with the tissue. In the above photograph, you can see some stains in places where the paint was not applied. This is from my first attempt at this project which I was not happy with so washed all of the paint off. The gouache stained the paint I think because the white paint had only been applied the day before and had not fully cured.
This is the final result:
