Expert blog
Sanding is mechanical surface treatment with abrasives. The essence of this process is in abrasion of a surface to roughen it to the extent needed. This can be done manually, but it is easier to use a sander.
Abrasives with Р80-Р180 grit size are used for paint stripping before applying putties on the surfaces. Generally, P80 is used for grinding down to a metal surface, while Р120-Р180 – for sanding the edges of a ground surface before application of putty.
There are the following types of sanders of different purpose:
Orbital or eccentric sander. This type of sanders is used for all activities, including paint stripping, grinding of damaged areas, levelling off of puttied surfaces, sanding of fast fillers and painted surfaces before application of other products. Eccentric sanders are multipurpose and have various strokes – from 2,5 mm to 10 mm.
One should know which sander stroke is used for this or that kind of repairs. Sanders with strokes from 5 mm to 10 mm are recommended for rough processing (surface grinding before application of putties, sanding of various types of putties), as such a stroke allows a bigger and coarser score mark.
It is advised to use sanders with strokes of 2,5-5 mm for finer processing (sanding of fast fillers and painted surfaces for further coating, surface treatment for further polishing), as this kind of stroke gives a smaller score mark.
Sanders with a stroke of 5 mm became widely used, as they may be successfully utilized for almost all types of auto body repair.
Soft interface pads should be used to avoid sanding through the extended parts while working with profiled pieces.
Preparation of a new part
Let’s talk about the preparation of a new part for coating
Generally, a manufacturer coats new parts with a special coat for transportation that protects metal from humidity in the atmosphere. This coat needs to be tested for chemical resistance against solvent. In case the solvent dissolves that coat, the coat needs to be removed from the surface mechanically. If the coat is solvent resistant, then it is enough to sand it with P400 abrasive using a specialized machine. For further working with the surface, it needs to be degreased thoroughly both after and before processing. This rule must be applied almost at any step of a car body repair process, but for degreasing of a puttied surface, because, as we’ve mentioned before, the putty is a hygroscopic material.
Sanding of puttied surfaces
The main aim is to remove an excessive layer of putty and to level off the surface. The sander is used only at the beginning of a sanding process or while sanding vast surfaces, the rest of the process is carried out manually with sanding blocks and planers. As we’ve described above, sanding of putties should start with coarse abrasives and end with more fine ones, but should not go beyond a step-by-step process. P240 abrasive should be used for finishing off a puttied surface for further application of a fast filler.
Sanding of fast fillers
Р360-Р500 abrasives are used for “dry” machine sanding of a fast filler for further coating of a surface, while Р600-Р800-Р1000 abrasives – for “wet” manual processing.
Sanding of plastic surfaces
Such sanding blocks as scotch-brite should be used for the preparation of plastic pieces for application of a basecoat. Then, following a technological process, one should utilize basecoats specially designed for plastic surfaces or special additives, namely plasticizers.
Sanding of painted surfaces
Abrasives with a P1500 and above grit size are used to eliminate defects of a painted surface.