Patching rust holes

You might need

Cut a piece of glass-fibre mat the same shape as the hole, but 1/4in. (6mm) larger, and lay it behind the hole. Brush liquid resin on to the mat until it is saturated. Lay a second layer of glass-fibre mat behind the first and brush more resin on. Let the resin set hard before final filling.

If a hole in the bodywork is not in a load-bearing area, such as a section of the chassis, it may be repaired with glass fibre, body filler or the more recently introduced filler-fibre mixture.

There are proprietary kits available which contain all the materials necessary for making repairs. Many of the materials can also be bought separately from accessory shops, garages or specialist firms.

Use an electric drill fitted with a wire brush, sanding disc or flap wheel to take off the paint for about 2 in. (50 mm) all round the hole. Wear protective goggles to guard your eyes against flying grit and paint dust.

Cut out any weak and rusted metal with tin snips. If any traces of rust remain, treat the metal with a rust preventative (See Eliminating rust before painting).

Bridging the gap

Cut a piece of the expanded metal to the shape of the hole, but about 1 in (25mm) larger all round. Put it under the hole and fix it in place with small lumps of filler containing extra hardener for quick settings. Add filler to build up the contour.

Tap the edges of the hole inwards with the ball of a small ball-pein hammer below the level of the panel.

Body filler can be supported by a sheet of expanded zinc metal fixed behind the hole. A suitable piece is usually supplied in the repair kit.

Use tin snips to cut the expanded metal so that it is about 1 in. (25 mm) larger than the hole all round.

Mix a small amount of body filler paste with the hardener from the filler kit. You can make it set quicker than normal by adding a little more hardener than the kit instructions suggest. But do not deviate too far from the maker's instructions.

Put the metal sheet behind the hole and fix it in position with small amounts of filler.

If you cannot hold the sheet from behind the hole, use wire hooks to pull it against the back edge until the filler that is fixing it has set or, depending on the type of hole, roll the mesh into a tube shape and fix it firmly into position. Add filler to build up the contour .

Laying up the glass fibre

Cut a piece of glass-fibre mat the same shape as the hole, but 1/4in. (6mm) larger, and lay it behind the hole. Brush liquid resin on to the mat until it is saturated. Lay a second layer of glass-fibre mat behind the first and brush more resin on. Let the resin set hard before final filling.

Glass fibre can cause skin irritation to some people - wear rubber gloves when you handle it.

Use scissors to cut out two pieces of the glass-fibre mat so that they overlap the edges of the hole by I in. (6 mm) all round.

Mix the liquid resin, following the instructions given with the kit.

Put a layer of glass-fibre mat in the hole from behind. Use an old paintbrush to dab the resin mixture on to the mat - both sides if possible - until the mat goes translucent.

If any resin drips on to the paintwork, wipe it off at once with a rag soaked in methylated spirit.

Put the second piece of mat over the rear of the first and add more resin with the brush. Let the resin set hard. Fill in the remaining slight hollow with filler (See Smoothing out a dent).

Using a filler-fibre mixture

Put the plastic sheet over the hole and tape down one side. Trace the shape of the hole onto the sheet with a pen.

Some repair kits do not use a mesh backing. They reduce the stages involved by using a plastic cover sheet to hold the combined filler and glass-fibre mixture to the same contour as the body.

Prepare the hole in the way already described under Bridging the gap (left).

Mix up a quantity of filler-fibre mixture and use it to pad around the edges of the hole until they are well coated. Fold back the plastic sheet and support it with one hand while you put filler mixture inside the traced outline.

Put the plastic sheet over the outside of the hole and fix one edge with a strip of paper masking tape.

Trace the outline of the hole on the plastic sheet, then fold the sheet back.

Fold the plastic sheet back over the hole, and press firmly to squash the filler-fibre mixture into the shape of the metal panel.

Add hardener to the filler mixture as directed by the instructions with the kit. Put the mixture on to the plastic sheet, inside the marked area. Dab more mixture on the edges of the hole.

Smooth the plastic sheet all round to match the lines of the panel being repaired and tape the other three edges to hold it.

Hinge the plastic sheet back over the hole, press it against the hole, and tape down the other three edges. The plastic sheet should follow the body contours.

When the filler mixture has set hard, remove the tap and lift off the plastic sheet. Fill in any small marks with a small amount of the mixture.

When the filler mixture has set hard, peel off the plastic sheet. Small marks left on the surface of the repair can be filled with small amounts of the mixture or can be filled with paintstopper before the repaired area is sprayed.

Filler-fibre mixture can also be obtained in cans, without plastic sheeting.

We also have this article in French and Spanish

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