Auto Body Repair & Body Primer Paint - Straight As A Pin

Blocking
As with the preceding bodywork, block sanding is a job that requires learned skill. We once again primarily used hard, flat sanding boards for the block sanding, both the long board and a quarter-sheet short board. Our preference in block sanding is to start out with a fairly aggressive cut, using No. 180 dry paper. The coarser paper will level much better than a finer grade, which will tend to "float" over flaws rather than cut. The guidecoat should be evenly wiped away as the sanding progresses, showing a level panel. Dark spots where the guidecoat remains indicates a low area, meaning the panel needs more sanding to get it perfectly level. If the sanding starts breaking through the primer before the low spot indicated by the guidecoat cleans up, the flaw is too deep and will need additional attention. Here a judgment call is made whether to use a filling glaze or another primer application, depending on the extent of the defect.

Masking is time consuming,...        read full caption Masking is time consuming, but effort here is well spent. We seal off everything, which keeps trash out of the primer and keeps the primer where it is supposed to be. Wheels off with the car elevated will make the lower areas and wheelwell lips accessible. Mask to the back side of the flange.

Solvent cleaning is done just...        read full caption Solvent cleaning is done just prior to spraying to remove surface contamination and oily residue. We used Sherwin-Williams Ultra Clean and a clean cheesecloth.

Three coats of Sherwin-Williams...        read full caption Three coats of Sherwin-Williams MP75 surfacer will provide plenty of fill and thickness for block sanding. The MP75 fills well and is excellent for this application.

After completing the block sanding with No. 180 grit, and any minor flaws are filled, we like to go in with a second application of surfacer. At this point, the surface should be nearly perfect and level, and the second application of primer is for texture, to fill the relatively rough sanding scratches. First, the car is detail sanded again, this time with No. 320 dry. Going over the surface with the No. 320-grit paper will reduce the sanding scratches that will need to be filled in the second primer session. Before the second primer application, we re-applied seam sealer as required at the hinge box and jamb seams, which had been removed when the car was stripped. The car is once again solvent cleaned, air dusted, and wiped with a tack rag. Because we were dry sanding, we did not remove the masking from the first primer session. Some painters prefer wet sanding, which requires removing the masking job and retaping. The second primer application is lighter than the first, since little filling is required. Two coats to give even coverage should be sufficient, followed by a fogging of guidecoat.

A light fog-coat of a contrasting...        read full caption A light fog-coat of a contrasting guidecoat is helpful for effective block sanding. The contrasting color will show flaws and high or low spots as the panels are being sanded.

We like an aggressive first...        read full caption We like an aggressive first primer cut, starting with No. 180 grit for effective leveling. Note how the guidecoat wipes as the panel is being sanded, showing the progress. The blocking with No. 180 grit is followed by a quick go-over with No. 320 grit to diminish the sanding scratch depth.

After a repeat application...        read full caption After a repeat application of MP75, a second block sanding with No. 400 wet gets the surface smoothed to perfection.

After the second primer application, the car is wet block sanded with No. 400-grit sandpaper. This final block sanding will provide additional leveling to the panels and a smooth silky surface for the finish painting to come. You know that it's perfect if the guidecoat washes away evenly in the final wet-sanding process, and the wet primer reflects a dead-level surface. This is the surface that great paintwork is built on and makes all the effort worthwhile.