Not wanting to derail Ben's thread, I'll just ride the off-topic train for one more station. There are only 2 rules to follow when applying sealants to paintwork:
Rule 1 - Never, never, I say NEVER apply a sealant over a wax.
Rule 2 - Verify full understanding of Rule 1.
Without a machine polisher to hand, it is safe to assume your C250 has accumulated its fair share of swirls and scratches over the last 6 years. Your car was sprayed at the factory with PPG Industries CeramiClear clearcoat. This is one of the hardest paint finishes to be found on any car so without some insanely monumental hand polishing effort, we have to accept those swirls and scratches are here to stay. Maximum gloss is all about flat surfaces and under a microscope your paintwork is an area of troughs of varying depth. Remember that physics lesson you slept through at school? The teacher was banging on about the fact that when light hits a flat surface, the angle of incidence equals the angle of reflection. If only they had just said "To get your Merc shining like a diamond in a goats ar$e, you need to WAKE UP!
So this is where the R222 Paint Cleaner (other cleaners are available) comes in. It is all about decontamination. Before applying your Fusso Coat you will want to remove all traces of your previous protection along with the tar spots and all manner of other crud you have accumulated just by driving around in your car. R222 will do this but additionally will "Polyfilla" in all those microscopic swirls and scratches to give both a flat surface for enhanced reflectivity and a contaminate and wax-free surface to which the Fusso Coat will readily bond. This is the very least you should do before applying any new sealant to a car that has been previously waxed. Purists will steer you to dedicated decontamination products and the need to "clay" the paint but without a machine polisher to do the necessary paint correction, we are essentially attempting to polish a turd.
Here's another link to a selection of Paint Cleaners, this time the prices are slightly higher but the P&P is free so psycologically it may seem a better deal. Not all contain fillers but each product has its merits so peruse them all.
I favour R222, Auto Finesse Tough Prep and the Poorboys products but your own particular set of circumstances will dictate your choice. The site also has an excellent Car Care Advice section which is also well worth a read.
Pre-Wax Cleaners and Glazes
Hope this helps,
Trev.