There’s been plenty of debate online lately about modified silicone (MS) versus standard silicone when it comes to building and resealing motorhomes and caravans. It’s the kind of discussion that gets people quite passionate – understandably, because the choice you make today can affect how easily your RV can be repaired five or ten years down the track.
I’ve spent 24 years in the motorhome industry, inspecting, repairing and advising on RVs of every shape and build quality you can imagine. Before that, I spent 14 years as a boatbuilder, working first in timber construction and later in composite (fibreglass) superyacht building.
Modified silicone versus standard silicone
Sealants and adhesives have been a constant thread through both industries, and I’ve seen first-hand what happens when the wrong product goes on in the wrong place. This month I want to answer the questions I get asked most often about MS versus standard silicone and hopefully add some clarity to a debate that can get more heated than it needs to be.
What is the difference between MS and standard silicone?
They’re both flexible sealants, but chemically they’re quite different animals. Standard silicone cures into a rubbery, inert material that bonds well to glass and some plastics, but it’s not a great ‘all-rounder’ for the mixed materials you find on an RV – aluminium, GRP, ABS, timber, powder-coated trims and painted panels all in the one seam. MS Silicone was developed to bridge that gap. It bonds to a much wider range of substrates, is highly UV stable, and – this is the big one for our industry – it can be painted over once cured.
Why does paintability matter so much for a motorhome?
Right: An example of sealant being applied like ‘thick icing on a cake’
Because motorhomes get bumped, resealed, upgraded and repaired far more often than a house or a boat hull. Awnings get replaced, roof vents get swapped, decals get changed, panels get repainted after a scrape. If a joint has been sealed with standard silicone, a repairer often can’t simply paint over the top of it – silicone leaves the surface unable to properly accept most conventional paints, causing what we call ‘fish eyeing’, where the paint pulls back into little rings instead of laying flat. MS doesn’t have that problem, so a repair or repaint can go ahead without a fight.
What happens if a repairer has to deal with standard silicone during a repair?
It can genuinely double the prep time on a job. Standard silicone has to be scraped back thoroughly, then the surface needs solvent cleaning and careful prep to make sure every trace – right down to microscopic residue – is gone before undercoat goes on. Even then, if any trace is left behind, you can still get fish-eyeing in the new paint. It’s not that it can’t be done – it’s that it takes real time, and time is what costs the owner money.
Does that mean standard silicone should never be used on an RV?
Not at all. This is where I think the online debate sometimes gets a bit black-and-white. Standard silicone still has legitimate uses, mainly for interior applications where UV exposure, paintability and future panel removal aren’t a factor. I’ve used it myself for certain interior jobs. The issue isn’t the product itself, it’s using it in exterior locations where it’s likely to be exposed to UV and may need to be removed, replaced or painted over down the track.
MS sticks to itself – why does that matter?
It’s a small detail with big, practical consequences. If you ever need to add a fresh bead of sealant alongside or on top of existing MS, it bonds properly. Standard silicone won’t do that reliably – new silicone applied over old often just sits on top rather than truly bonding, which can create a false sense of security about how well sealed a joint actually is.
Is thicker always better when it comes to sealant beads?
No, and this is one I’d love to put to bed. A neat, properly sized bead does the job just as well as – and usually better than – an oversized one. I do a lot of inspections on Australian-built vans, and it’s common to see very generous beads of standard silicone along roof seams, J-mould joints and around roof-mounted fittings. There’s a well-known Australian caravan repairer with a big social media following who’s a strong advocate for this approach, and he gets plenty of engagement (for and against I might add) whenever he posts about it.
I understand why people may think; ‘more sealant, more protection’. Vehicles built that way aren’t necessarily poorly made. It’s just a different philosophy to what’s become standard practice here in New Zealand, where most builders and repairers favour a leaner, MS-based approach because of how well it suits our conditions and our repair culture.
Does more silicone actually mean a better seal?
Not really. Once you’ve got proper coverage and adhesion across the joint, extra thickness isn’t adding meaningful protection – it’s just adding more material that eventually has to be dealt with if that joint is ever opened up again.
Does the amount of silicone used cause a problem if nothing ever needs to be removed?
Generally, no. If a panel or fitting never needs to come off, a generous bead of standard silicone can sit there for years without drama. The problem only shows up the day someone needs to remove a part, do a repair, or repaint – and on an RV, that day tends to come sooner than owners expect.
Last month, I saw exactly that play out, when a repairer spent nearly four hours with a plastic scraper and a tub of solvent, just to get one roof seam ready to repaint. Not because the panel was damaged – because of what was underneath it. A bead of standard silicone laid on thick, that had to come off millimetre by millimetre before any paint would stick.
What about UV stability – is that really a factor in New Zealand?
Absolutely and it’s one of the main reasons MS has become the standard choice here. New Zealand’s UV levels are among the highest in the world, and any sealant left exposed on an exterior seam is going to spend years baking in strong sunlight. MS holds up extremely well under those conditions. That’s a genuine, practical reason it suits our market, quite apart from any argument about adhesion or paintability.
I’ve heard people argue standard silicone is ‘more flexible’ – is that a real advantage?
It comes up a lot, and it’s true that standard silicone can have excellent flexibility. But I’d ask: where on an RV do you actually need that degree of movement? A motorhome does flex along its overall length and chassis – particularly out on unsealed roads – but that flex is distributed across many metres of structure. Over a joint that might only be a couple of hundred millimetres long, the actual movement is minimal. MS provides more than enough flexibility for that scale of joint.
What about silicone-based roof coatings – same issue?
Yes, and it’s worth mentioning because I still see it causing headaches. There’s a fairly popular silicone-based roof coating product used here in New Zealand. It has the same fundamental drawback as standard silicone: nothing sticks to it afterwards. It’s not paintable, and while you can peel sections of it off with your fingers, it leaves behind a residue that’s genuinely difficult to fully remove. That makes a future repaint a much longer, more expensive job than it needs to be.
Have you used silicone yourself on an RV?
Yes – but only ever for interior repairs and for some plastic window installations where it’s the correct product to use. I wouldn’t apply standard silicone to the exterior of a motorhome or caravan, based on what I’ve seen happen down the track. That said, I’m not suggesting every van built with more generous use of standard silicone is a problem waiting to happen – it comes down to where it’s used, and what a future repairer will need to do to work around it.
What’s your overall advice to owners and buyers when it comes to silicone?
Understand what’s been used where, especially before you buy an RV or commission a repair. If you’re building or resealing, ask your builder or repairer which product they’re using and why. In most cases here in New Zealand, MS will be the sensible choice for exterior work because of our UV exposure, our repair and repaint culture, and the simple practicality of being able to paint over it without a fight. Standard silicone still has its place – just make sure it’s the right place.
As always, if you’re ever unsure what’s gone into your van’s construction or a previous repair, get it checked before you commit to buying, repainting, or handing it over for work. It can save a lot of time, money and frustration later on.
Andrew Tapp is a veteran of the motorhome and boatbuilding industries. He converted his first 9m Hino in 2002 before designing his own series of luxury motorhomes. He has worked on all classes and types of RVs, motorhomes and caravans and has seen it all. He now puts his decades of experience to good use, providing Pre-Purchase Inspections, BubbleProof Pressure Testing, Water Ingress Testing and RV Health Checks.
