The reason that you have bubbles between and behind component legs is probably due to the viscosity (high) and thickness (100²m +) of the material applied.
What tends to happen is the coating dries on the surface and solvent is trapped in the coating, if this coating contains solvent. This is also probably accentuated by the fact the coating is not flowing under the device due to high viscosity and is trapping air under the device.
I would suggest a water based coating, acrylic or polyurethane, if you need low or no VOC, or a solvent based coating if solvents are not an issue for you.
The lower viscosity combined with a reduction in coating thickness will eliminate the bubbles.
I presume the original boards are coated with silicone, if not I would only suggest using a silicone coating if the units that you coat have a continuous operating temperature over 150°C.
Below 150°C and if the original boards are not coated with silicone I would suggest acrylic if only moisture protection is required and polyurethane if moisture and chemical protection are required.
Wednesday, 17th March 2010