Blocking 101: Smoothing And Shaping The Surface

When you run your fingers over a great paint job, you not only are witnessing the efforts of a great painter, but also the person who spent the time blocking the vehicle body prior to paint. Frankly, you can’t have a great paint job unless you have blocked the primered surfaces to perfection.

A block for every situation – this entire assortment of blocks was purchased for under $40 and can be ordered either on line or purchased at your local paint supply house. Never block you vehicle without a sanding block of some kind. Some old school folks feel they can block a car without a sanding block.

Sanding scratches and wavy panels are the result of poor blocking techniques – an art that is simply just not that hard to master.

Assessing The Situation

For this ’67 Chevelle, the body was already in pretty good shape, requiring no “real” bodywork like patching panels, grinding out cancerous edges and corners or the like; just a limited few dings and small dents.

After DA sanding the body for a first pass of sealing primer, we applied the first light coat. Here is where the blocking begins.

Starting with 220-grit paper, we selected a proper shape sanding block and proceeded to smooth the surface.

The key here is to use a lot of sandpaper (changing it often) and keep the surface as clean as possible by wiping down the body often with grease and wax remover.

Sandpaper for blocking can be purchased in a variety of grits. Mark Oja, owner of Custom Rod Garage of Southern California, recommended use of 220-grit sandpaper for the initial cutting of the primer. For the final surface prep, 400-grit paper is used.

These adhesive backed papers grip the block and are easily removed. Make sure the paper covers the entire block. The block should never be exposed to the paint or primered surface. Blocking paper can also be ordered in a roll and trimmed to fit the block. Make certain the sandpaper lies flat on the surface of the block without wrinkles or bubbles that can alter the contact surface between the paper and the primered surface.

Clean, Clean, Clean

Blow off the area with compressed air and then sweep up the debris on the floor so it ensures a clean work area. For a contaminant-free paint job, you need a surgically-clean work area.

Sanding scratches is an art that is simply just not that hard to master

In most cases, you will want to primer the body at least two times before even thinking about applying paint. If flaws are detected in the primered surface, a little spot putty can be applied and sanded smooth, covered with primer and blocked again. With each successive blocking session, the sand paper used should become smoother, moving up to 400-grit before the painting stage.

A guide coat (light spraying of a contrasting color to your primer) will help show were you have been and make certain you don’t miss any areas when blocking the vehicle. Use you fingers to feel flaws in the surface. That is the best tool you can use to ensure a proper finish. Remember, the time you spend here will really pay off in the end.

When blocking a surface, you should move the block in a crossing pattern making a big X on the surface. Be careful when you approach the edges of a panel or corner. They key is not to burn through the primer and expose the lower layers. If you burn through the primer to the metal, make sure you reprimer and cover these areas. Applying primer to raw metal is a big no-no. The crossing pattern action will generate a great amount of sanding debris. Clean the paper by knocking out the loose debris every few minutes. If there are any chunks of dirt in the paper, change it. Then change the often – using old,dirty paper can cause scratches in the surface and make the job that much harder.

Regardless of the area being blocked, round, flat or curved, the crossing pattern should continue when you block the vehicle. Note how the sandpaper adheres to the round block.


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