Any tips on "heat-smoothing" plastic hulls?

[SWriverstone]

As with everything on earth, there are a ton of videos on YouTube about ways to "renew" the hulls of plastic boats (when they get scratched-up, have tiny strings of plastic hanging off, etc.) Everything from sanding with progressively finer-grit sandpaper...to using irons, propane torches, heat guns, etc....to filling deep scratches with melted clorox bottles, coat hangers, or any other piece of HDPE junk you can find.

First let me say two things:

1. This likely is purely cosmetic and not necessary at all (so I may just forget about it). People like to say "a smooth hull goes faster," but unless you're an Olympic racer where hundredths of a second matters, this is nonsense. Scratches will make zero noticeable difference at the speeds most of us paddle.

2. I'm well aware that you can ruin your boat if you aren't careful! (People love to tell stories of folks melting holes in their hulls, LOL.)

All that said, has anyone done anything like this? Did it work? How did you do it? (And did a shiny, smooth hull make you feel better? LOL)

Scott

PS - I know that all plastic boats aren't made of the same kinds of plastic. Or are they? (People often differentiate between "special" plastic, like whatever P&H uses...and run-of-the-mill plastic, like Wilderness System uses. I figure it's all plastic!

 

[Kayak Jim]

I had a Prijon Seayak (which is injection molded PE not rotomolded like most) and considered filling a couple of the deepest scratches using a p-tex "candle". These are sold to repair ski bases and I did repair a few skis with them. You light the end of the pencil shaped candle, then let it drip into the crack. Then scrape off the excess. Worked well on the skis but I don't think I ever got around to doing the kayak. Gave me an excuse for paddling at the back of the pack.

 

[CPS]

When I had a rotomolded boat I'd occasionally go over the hull with a razor knife and trim off the largest of the ridges near scapes. Not really for speed or looks, but it did eliminate spots to scrape my knuckles on while loading the kayak onto my roof. Particularly in winter paddling when my hands are cold, sensitive, and mushy.

If you want to fill a deep gauge, or need some material for welding up a small crack, I would recommend trimming a bit from around the hatch openings. Usually there is a pretty large lip there which can be a suitable donor. Of course if you don't care about a colour match you can use whatever suitable plastic is around.

I would note that while HDPE (high density polyethylene, AKA #2 plastic) seems to refer to one particular chemical composition, there are subtle differences out there. So the plastic from a milk jug, or Rubbermaid tote may bond, but may not exactly behave the same way. It might be less flexible, more flexible, easier to gouge, etc.
Even different colours have subtle variations in how much they expand at various temperatures. This has the (possibly) interesting result that 2 rotomolded boats made in different colours which are pulled from the same mold would be ever so slightly different dimensionally.

Cross-linked poly boats were experimented on for a while, but I think most manufacturers have moved away from them. Those were unable to be welded as the cross-linking process changes the way the plastic behaves at heat, instead of getting soft as it warms up it gets harder, then lights on fire. Not a helpful attribute and presented additional environmental concerns. Something to consider for older kayaks, particularly whitewater circa 1990.


Anyways...

When I've done it I've usually warmed the surrounding material up with a heat gun, then using a fine tipped torch melted donor material into the area requiring new material. Then kind of mix the old and the new together to promote adhesion. I usually used the back of a spoon to shape things a bit and encourage heat to go into specific areas. Having a wet rag at hand is helpful. The melting point and flash point (where fire happens) have a decent spread between them so if you're careful you might be good.
A better option would be to use a plastic welding tool with a temperature control on it. A soldering iron also works, especially if it accepts flat blade type tips.

 

[SWriverstone]

Thanks @CPS - I figured you might have had some experience with this. As I mentioned above, I'm uncertain whether I even want to do this? The scratches on my WS Tsunami 165 are totally within the "normal use" category (no huge gouges). And I already know I can paddle this boat (empty) at 5mph/4.3kts, with scratches, which seems fine.

I've also noticed lots of ideas online for getting rid of oil canning in the hull. My Tsunami only has very mild oil canning in a couple spots—and again, it doesn't seem to have impacted the boat's speed at all. (Some of the techniques are wild, like pouring gallons of hot water inside the boat to heat the whole hull around the deformed spot—that sounds like a hassle!)

I'm thinking the best approach with plastic boats is to embrace their imperfect-but-rugged "plastic-ness."

 

[CPS]

The most effective method I'd seen for removing oil canning is to place a dry towel over the dent, then pour a kettle of boiling water on it. The towel will absorb the hot water and hold the heat in place. Often that's enough to get a mild dent to pop.

For stubborn dents or really deep oil canning from years of improper storage I do the same process, but first will insert something to put some pressure on the backside of the dent before applying heat. My preference is scraps of 2x4 and chunks of minicell foam. You can also heat the area and press on it from the other side, and hold the pressure while it cools, but that's annoying.

 

[tinman]

Hmm, I tried removing a dent from the bottom of the blue boat in the photo some time ago. Caused no doubt by storing it on the ground in all weather conditions. A good ol rotomolded boat from the 1990's. I hung it in the air by rope from the handles then put a couple of bricks inside it on top of the dented area, then took a space heater and positioned it underneath. Not to close and not too far, just enough to warm the area. I dont think it helped much really but it was fun trying. The dent is still there but since I only bring it out of retirement when taking friends along with me for short introductions to kayaking I havent worried about it too much.

 

[mick_allen]

I've done a little re-modifying with a heat gun and my take away for oil canning is that gravity is not your friend when an area is heated up.

I would only attempt it - if it was bad - and then I would back the whole area up so no sagging could take place, heat it, and quench it quickly to see if the area contracted thereby lessening the oil-can: like I wonder if you could do it in mini-steps?

Otherwise an alternative would be to always use blow-up airbags in the hatch areas to both minimize the canning, provide extra floatation backup, some strucural backup, and if zipped - extra gear protection.

Or the other approach is to take a leaf out of some plastic manufacturer's books [early and late] and place in appropriate chine logs, rib logs, or surface logs [wood or foam glass covered]. If it's just for oil-canning, structure or longevity is not a concern and so glue them back in if they ever slightly pop free. Many, many manufacturers do this and if your kayak starts showing poor aesthetic effects, those manufacturers lack of foresight now need a little help from you! Maybe ask for a few bucks back or you'll start showing [on places like this] how you had to deal with all the defects.

 

[SWriverstone]

In my experience, oil canning most often results from carrying a plastic boat on racks (even cradles) upright. For years I've been carrying all my boats (composite and plastic) on their sides. (Same for storing them as well.) This is the strongest part of the boat, and takes the load off the hull. So far it has worked well. I barely ever get oil canning...but sometimes with rotomolded boats I think a bit of it is unavoidable.

 

[tinman]

In my experience, oil canning most often results from carrying a plastic boat on racks (even cradles) upright. For years I've been carrying all my boats (composite and plastic) on their sides. (Same for storing them as well.) This is the strongest part of the boat, and takes the load off the hull. So far it has worked well. I barely ever get oil canning...but sometimes with rotomolded boats I think a bit of it is unavoidable.

Yup, which is why I now store my kayaks on their side as evidenced by <--------------