Putty and body filler both fill in body indentations, but there are some key differences between the two. Putty is air hardening and used to fill in marks and scratches on body filler. It is usually very similar to toothpaste-thick oxide primer.
Putty filling can be easily sanded if it has not been mixed with body filler. However, putty that has small pieces of chopped fiberglass will not sand down as well or stay as flexible as non-chopped pieces. Body filler is also much thicker than putty, and it can be mixed perfectly with chemicals for body work. This increased thickness makes body filler a structural material rather than just a thick paint like putty for scratch repair.
Putty remains soft for only about an hour after it has been mixed, whereas body filler will remain workable for up to 24 hours. One advantage of putty over body filler is that it is generally much less expensive than body filler. However, if too much surface area is covered with putty, then corrosion could appear under the paint.
Body filler is sealed with body sealer, which helps to prevent rusting from occurring under the paint.
What are some uses for body filler and putty?
Both are used in small dents and body indentations. However, body filler is used primarily for small dents that must be repaired with an exterior body, whereas putty might only be used in places where the body will never be exposed to the outside elements.
The general rule for repairing a dent with either body filler or putty is that the area of repair should not exceed 1 square foot. This is because after about 1 square foot, body filler and putty can no longer be easily sanded down without affecting the surrounding paint job. Another difference between body filler and putty is that body filler can become a structural part of a car, whereas putty often fills in the scratch once and then it may need to be touched up again depending on how many times someone opens the body of the car.
However, body filler can also crack if it is not sanded or primed properly after application.
Putty should be used for filling in small dents because body filler is generally much thicker than putty and may need to be mixed with chemicals which make it difficult to apply to body indentations. For this reason, body filler should really only be used to fill in larger dents.
Another difference between putty and body filler is that body filler often comes premixed whereas putting does not come premixed and must be made by mixing together various powders like cornstarch, talcum powder, resin, polyethylene glycols (PEG), calcium carbonate (CaCO3) or lime which are all mixed together in a body filler kit.
Take a look at our article summarising the best car body fillers on the market.