After sanding down to the finished level, you may find that there are cavities that have been created by air bubbles that were trapped in the holes or gaps. After drying the work-piece thoroughly and wiping over with wax remover, one remedy is to repeat the previous step until the surface is uniformly smooth and even. Another answer, especially for small pinholes, is to use a product called spot putty. Spot putty in essence is a premixed body filler of thinner consistency. Again, thin layers are better than big blobs. As you can see to the right in the image below, there shouldn’t be any residual body filler above the level of the holes, or on the surface of the panel. 4
Spray putty will fill in any small scratches and blemishes on the prepared surface, and applies like a thick undercoat. To get an absolutely perfect final finish, such as the mirror finish on show cars, can take as many as 30 individual coats of filler, with each coat almost completely sanded off before the next is applied. Sanding back panels should only be done with 800-grit Wet’n’Dry, plenty of water and a sanding block.
Spray putty can initially be applied in spot areas and then once all of the problem areas are looking as you want them, the whole panel can be sprayed and sanded back with 800-grit Wet’n’Dry. 5
Undercoats, or etch coats, are important from several standpoints, and the word of the day here is ‘Consistency’.
Firstly, body filler and spray putty are porous, which means that they will absorb paint at a different rate to the metal or pre-painted areas of the panel. They also have a tendency to dry to a matt or semi-gloss finish. Undercoats seal these compounds and provide surface consistency.
Undercoats are also important as they lock the paint to the surface below. Imagine it like a chemical version of Velcro. Most etches especially work in this way, eroding or melting the surface underneath to create that bond as they cure (or dry). The result is a consistency in adhesion.
The third important attribute of an undercoat is to give the object a consistency of colour. The image below shows the side cover after one light dust coat of etch, and it is still easy to see the holes and spray filler – it might take several coats of a light-coloured paint to cover it up satisfactorily. An undercoat will give an even-coloured canvas to apply the final coats over.
Each application of undercoat should be allowed to dry and then gently sanded back with 800-grit Wet’n’Dry. 6
When it comes to the actual process of painting there are quite a few options available, depending on the amount of cash you want to spend and the finish you’re looking for. The best results will be from using automotive finishes sprayed at high pressure in a hermetically-sealed room, and then dried under heat-lights. Not that practical for most home handypersons! Of course, it is possible to take your primped and primed case to a professional spray painter for the final squirt, and if you are not fussy as to the colour they might even offer to ‘blow some colour over it next time we are painting a (insert colour choice!) car’, usually at a very reasonable cost.
In the case of most DIY projects, the next best answer is to use automotive touch-up paints, which are widely available in a multitude of colours and relatively inexpensive in small quantities. One of our favourites is the Holts Dupli-Color range of ‘mixed for you’ aerosols, allowing you to choose colours that may not be available commercially.