In addition to tape masks, further masking techniques for holes and complex geometries are used, as shown in the figure. Radiused corners are protected using curved masks to keep unmelted particles and other debris from contaminating the coating.
Dowel and screw holes can be masked using simple tapered silicone rubber plugs whose top surfaces are above the surface being coated and a slightly larger diameter than the hole, so tapered, low-stress edges will result.
Masking compounds are useful for providing short-term protection from overspray but are not useful for protection from grit blasting. Commercially available compounds comprise refractory particles, such as graphite, carried in water- or solvent-soluble binders. These can be applied by brushing or spraying.
When dry, the film prevents overspray from sticking, while the refractory particles provide short-term heat protection. Excessive exposure to the spray stream, however, will cause these compounds to break down, after which spray particles may stick. Residual compound is easily removed by rinsing with water or solvents, depending on the carrier solubility. Masking compounds are easy and convenient to use in spite of their shortcomings. Epoxy/puttylike compounds are also suitable for plasma and HVOF masking needs.
Image – Examples of how to use masking techniques for special configurations
This information is from ASM Handbooks Online, Vol. 5A: Thermal Spray Technology, Coating Structures, Properties, and Materials. To find this information (subscription required), click on the link below and scroll down to Figure 11.
