Best Sealant for Caravan Joints

What is the best sealant for caravan joints on painted aluminium, Zincalume and plastic?

Selleys ProSeries Flexiseal. It is a polyurethane sealant, similar to those commonly used within the caravanning industry, and it is a highly flexible sealant with excellent adhesion. It is suitable for use on most water-based paints.

The reason a polyurethane is the industry choice for vans rather than a silicone comes down to three things a caravan joint demands at once: adhesion to painted metal, tolerance of constant vibration and thermal movement, and the ability to be painted over. Selleys Flexiseal is a non-slumping, low modulus polyurethane joint sealant with ±25% joint movement capability, excellent resistance to ageing and weathering, and no shrinkage during curing. It is supplied as a 600mL sausage in a range of colours.

Your three substrates, one at a time

Painted aluminium. Flexiseal has excellent adhesion on metals including aluminium, and it is suitable for use on most water-based paints. But the qualification matters: for painted metal and pre-painted surfaces, a pre-test to confirm adhesion is recommended before full application, and priming may be required in some cases. Caravan paint systems vary, so bond a short test bead in an inconspicuous spot, let it cure, and try to peel it before you seal a whole roof line.

Zincalume sheet steel. Metals are among its listed substrates. As with the painted aluminium, clean the surface thoroughly — Zincalume carries a manufacturing oil film when new — and pre-test if the panel is coated.

Plastic. This is where you need to be careful. Flexiseal adheres to some plastics, and pre-testing is recommended for certain plastic types. It is not suitable for polyethylene (PE) or polypropylene (PP), and it will not adhere to those at all. Identify the plastic before you seal to it; many caravan trims and mouldings are polypropylene.

Joint dimensions

Polyurethane sealants perform according to how the joint is shaped, and Flexiseal publishes its requirements:

• Minimum joint width and depth: 6mm.
• Joints up to 12mm: use a 1:1 width to depth ratio.
• Joints greater than 12mm: use a 2:1 width to depth ratio.
• Optimum width to depth ratio: 2:1.

A bead that is too deep for its width cannot stretch properly and tears at the bond line. Install a backing rod to set the correct depth on anything wider than about 12mm.

The compatibility rule that ruins caravan resealing jobs

This is the single most important thing on this page. Polyurethane and curing silicone do not mix.

Flexiseal should not be applied or cured in the presence of curing silicone sealants or curing polyurethane sealants, because they can interfere with its cure. The reverse also holds: Selleys Roof & Gutter Silicone may not dry or cure properly where a polyurethane sealant has been used previously.

Caravans are routinely resealed over their lives with whatever was on the shelf, so an old joint may well have both chemistries in it. If you are stripping an existing seal, remove it completely, and do not run a silicone bead alongside a fresh polyurethane one.

How to seal a caravan joint

1. Remove the old sealant completely. Cut and scrape it out; do not bead over the top of it.
2. Clean the joint so it is free of oil, grease, release agents, dust and loose material, and let it dry.
3. Pre-test adhesion on the painted or pre-painted surfaces and on any plastic you are unsure of.
4. Install backing rod to the correct depth, and mask either side of the joint if you want a clean line.
5. Cut the end of the sausage and insert it into a Selleys sausage gun. Extrude into the joint so the sealant makes full contact with both sides.
6. Smooth with a spatula, forcing the sealant into the joint for better contact with the sides.
7. Remove the masking tape before the sealant skins.
8. If painting, use water-based paints after curing. For a solvent-based paint, apply a water-based primer first.

Limitations

• Not suitable for polyethylene or polypropylene.
• Pre-testing required on certain plastics and on pre-painted surfaces.
• Do not apply or cure in the presence of curing silicone or polyurethane sealants.
• Not recommended for continuous submersion or below water line use.
• It is moisture-cured, so it needs sufficient exposure to atmospheric moisture.

Common questions about caravan sealants

Why a polyurethane rather than a silicone?

Polyurethane sealants are commonly used within the caravanning industry, they adhere well to painted metal, and unlike silicone they can be painted over.

Will it stick to painted aluminium?

It is suitable for use on most water-based paints and has excellent adhesion to metals, but pre-test on your specific painted surface first — priming may be required.

Will it stick to the plastic trims?

Some plastics, yes; polyethylene and polypropylene, no. Pre-test before you commit.

Can I paint over it?

Yes, with water-based paints after it has cured. Use a water-based primer first if you want a solvent-based topcoat.

How much movement does it take?

±25% joint movement capability, and it is certified to ISO 11600/F/25LM.

Can I use it over an old silicone seal?

No. Curing silicone can interfere with the cure of a polyurethane. Strip the old sealant out completely.

What size joint does it need?

A minimum of 6mm wide and 6mm deep, with a 1:1 ratio up to 12mm and 2:1 above that.

What packaging does it come in?

A 600mL sausage, in a range of colours. You will need a sausage gun rather than a standard caulking gun.

Can I use it below the water line on a boat?

No. It is not recommended for continuous submersion or below water line use.

Is there a guide to caravan work?

Yes — resealing a seam in a caravan and how to make over your caravan.