White can be tougher to color match than you might think. On a 124 you would have either artic white (147) or polor white (149) - it should be on your data plate. Artic white was a single stage urethane while polor white, on later cars, was a basecoat / clearcoat. Any air dryed paint, that is from a spray can, is going to be significantly more susceptable to stone chips. It just never gets very hard. Only with a catalzyed urethane will it be very tough. A flex agent additive will make it even more chip resistant. But all this - paint, reducer, catalyst, flex agent - is probably going to set you back at least $50 to $75 even for the smallest quantities. A Preval spray bottle, another $5, and a little wet sanding should get you a smooth finish. Touch up spray cans are usually laquer, the least durable of all auto paints. If you decide on a spray can put it on thin, with white primer - thin is less prone to chip. Another alternative is to drop them off at a shop that does a lot of Mercedes and have them sprayed next time they have your color already in the gun - for artic white this should be pretty regularly.