Sill damage: repair or replace

What the sill is

Two owners in 2009 agreed that the sill panel is fibreglass, bonded to the tub, and is mainly cosmetic. The structure is the aluminium extrusion you can see on the inner sill. This is the owners' understanding, not something from the factory manual.

Repair or replace?

• Repair is usually the first choice. An owner with impact damage in 2009 was advised by several repairers not to replace the sill, because the work is "horrendous" and the car can squeak and rattle afterwards where it goes back together. A classic-car specialist repaired it in three days, against weeks for a replacement. He was quoted about £2.5k to replace and about £800 to repair.
• If the damage is on an insurance claim, the insurer may insist on a replacement panel. Another owner said an insurer will not demand a replacement. Opinions differ, so ask yours.
• A repair section cut from a reproduction sill is an option for local damage (suggested by several owners in 2009 and 2019).
• Replace when the sill is too far gone. One owner who replaced both sides fitted a used genuine one on one side and a reproduction on the other. They cost the same, and the reproduction needed far more fettling.

Typical damage

• Crack along the line where the door meets the sill (2011): a 6 inch crack after someone slipped on the sill. Opinions on the repair: a bodyshop cuts a hole under the sill, repairs the top invisibly, then repairs the hole underneath; about £500 for a perfect repair (one owner). Another said £200 many years earlier, including a metal plate underneath to strengthen it. Another said the repair is hard because you can't get to the underside, and that a rigid repair on a sill top that flexes can crack again.
• Holes from jacking in the wrong place: one owner had both sills repaired for about £500, and then a piece of debris put a hole in one again two months later.
• Impact damage to the front corner (2009): a chunk of fibreglass knocked off ahead of the door. One owner said it is non-structural but bonded to the tub, which is why a replacement is such a big job.

Expanding foam behind a crack was suggested and rejected: it would need to fill the whole void to bond to the underside.

A DIY repair (2013)

One owner repaired two badly damaged sills himself, one with a blow-out along about two thirds of its length where it bonds to the chassis. These are his steps and advice, not a tested procedure.

1. Cut out the damaged area.
2. Lay up fibreglass sheet to match, and cut it to size.
3. Bond it in place using fibreglass bonding paste and tape.
4. If you can reach the inside, patch the join with woven fibreglass there.
5. Fill gaps and changes in height on the outside with body filler, then skim with an easy-sand filler, sand flush and fill with putty or filler-primer.

• He first used aluminium mesh to hold the shape across a gap, then glassed over it. Another owner suggested laying up a flat sheet and slotting it into the hole, so the fresh glass bonds to the sill, with self-tappers to pull it together while it cures.
• A third owner said to laminate strips of light glass on a flat surface, peel them off once cured, and glue them in behind the damage. Secure them with self-tapping screws or a temporary plate on the outside, then drill small holes so the filler keys.
• Where the sill butts up to the tub he first tried a patch, which looked poor, and then cut out the area and rebuilt it as above.

Paint (one owner's method, 2013)

1. Key the old paint with 240 grit. Don't take it back to the gelcoat: paint sticks to paint better than to gelcoat.
2. Fill stone chips and small damage, and sand with 240.
3. Two coats of high-build primer.
4. A guide coat (1 part primer, 1 part base coat, 6 parts thinner), sanded back with 400 grit to remove the high spots.
5. Base coat, then clear coat. No sanding between coats on metallics, because it removes the metal flake. On solid colours, wet-sanding before clear was suggested by another owner.

He used cellulose for the clear coat, and noted that cellulose is not petrol-resistant. Only two-pack paints really hold up to fuel. A rattle can clear coat may need cutting and polishing, and an owner with a gun suggested a light dust coat first, then a full wet coat.

Replacing a sill

• Both clams and the door have to come off (2019, from an experienced owner). It isn't difficult, just very time-consuming. See front clam removal and rear clam removal.
• Do a dry run before bonding it on, so the clams line up.
• Paint the new sill before it goes on. Have plenty of rags ready, because the sealant gets everywhere.
• A reproduction sill needs finishing work on the edges before it will fit.
• Video tutorials from 2008 were mentioned, but the links are dead.