Repairing Plastic Water Tanks

Posted by Drew Frye at 09:20AM - Comments: (4)

Tester Drew Frye has some assistance while repairing a polyethylene fish pond as part of long-term testing.

Plastic holding tanks and water tanks have the advantage of not corroding, but they are hardly failure proof.† Plastic holding tanks can crack and fail under extreme vacuum pressure if the vent is clogged. An errant drill or saw used to make adjacent repairs can puncture a tank. Or a poorly secured tank can be hurled across a cabin and crack.

In our recent report on polyethylene (PE) tank repairs, we explored several different methods of repair, including heat-welding using commonly available welding kits. As that article points out, the manufacturers of these tanks do not recommend such repairs. However, our long-term tests of reinforced water and holding tank repairs (not fuel!) have so far been very promising.

A typical plastic repair kit from Harbor Freight.

Hereís a generic look at the repair procedure using one of two plastic repair kits we tested. We also had limited success using epoxy and polyurethane adhesive, and these repair procedures are outlined in the main article, which appears in the December 2018 issue.†

The first step for all cracks is to drill a stop hole (3/16-inch is usually adequate) at each end of the crack to prevent it from spreading. If your hole is more than superficial, youíll usually need a filler strip.

One source suggested trying a milk jug for filler, as it had been used by others. Although the milk jug approach might work in some cases, in our experience, the thin strips tend to fall down through the crack instead of building a bridge. The repeated heating required to lay down enough material often made the crack wider.

It was easier to work with the thicker strips that came with the supplied repair kits. We used these for our test.

Prep work. Clean and prepare the area around the crack with 60-grit sandpaper. If the hole or crack is very small, use the plastic welding iron to melt plastic around the hole and fill it in, being careful not to melt through the tank. Melt only a thin layer and work it into the crack.

It is very important to heat-treat the repair area to ensure a strong, consistent bond.

Heating. Next, use the welding iron to soften the area around the repair to insure good adhesion, soften one side of the filler strips, and stick them on, apply 1-2 layers, building up until you match the thickness of the tank material, tapering for five tank thicknesses wide on every side (at least 5/8-inch past the crack).

Melting the filler material with the 85-watt Harbor Freight tool welder took longer that it would with the stronger (200 W) professional model.

Blending. Smooth the layers of plastic together, but do not linger too long in one area, or the heated plastic will begin to sag into the tank. Keep in mind that your aim is not a true fusion weld, as in steel welding when the filler and base material are well mixed. This simply does not work. If you tried to apply enough heat to fully liquefy the PE, it would shrink away from the crack, and fall into the tank before you finish. In addition, the melting points of the filler and the base material, even if both PE, are probably somewhat different.

Stainless mesh makes for a much tougher repair.

Reinforcing. Incorporating reinforcing material makes the repair stronger and tougher. We recommend this for both adhesive and welded repair, but the materials are different. We tried using fiberglass cloth with melted polyethylene, but it was hard to keep in place during welding.

We settled on stainless steel mesh. This is included in welding kits, and you can also get small pieces in the plumbing department, sold as lint screens. Cut a piece about 1-inch wide and 1-inch longer than the crack. Soften the tank surface with the welding iron and then press the mesh into the plastic. Do not press too hard; wait for the plastic to get soft and it will settle right in.

If the surface is curved, cover a small area, let it harden, and then work the mesh into the plastic in stages, bit by bit, as if fitting a replacement plank or bending a steel hull plate. Once the mesh is set, lay filler material on top of it. Pre-soften one side of the filler material with the iron, press it onto the screen, and use the iron to melt and smooth it down. Cover the mesh with several layers of filler, continue building up to about 1/8-inch. There is an excellent how-to video by Polyvance, the makers of one of the tested kits.

Repeated flexing caused some cracks to appear in the reinforced weld, but testers could not break the bond.

Sanding. After the filler and tank cools, you can sand it smooth if you have added enough filler material. The sanding will leave it dull and fuzzy, but a light flaming with a torch, similar to how you prepped the area, will restore some of the gloss. If you need to match color, use strips of matching PE. The color worked well with our kayak repair, but for a tank repair you can skip these cosmetic steps.

Practice. It is possible to make things worse. If you watch enough You Tube repair videos, you will see people make small cracks into big holes by applying too much heat or using filler material with too high a melting point. We made the same mistakes during practice, before making the welds we used for testing. A 5-gallon bucket, a kidís playhouse, or a food storage container makes for a good test bed. Try to find something similar in thickness to your tank.

Comments (4)

Good article, I have a 81 Morgan and one of the water tanks had a 9 inch crack at a baffle. After doing some reading I bought a welder from Harbor Freight . My first of many attempts was with cutting up a 5 gallon bucket for filler material. I would than fill it with water and would always find a drip. I chased that drip for hours and finally gave up. I found in some cases I could peel the material off. I later called the company that made the tank and asked what material the tank was, it turned out to be LDPE not HDPE. Once I bought the right welding rod and a separate shoe attachment with a feed hole for the welding rod I was able to repair the tank the first time. The shoe made a big difference. I also bought a stainless food strainer and removed the screen and used that as reinforcement. Not a leak since.

Posted by: Rick Dowe | December 1, 2018 4:00 PM    Report this comment

As a plastics engineer (yes that is a real degree) with almost 40 years experience, there are certain truths. One is: "Like, Likes, Like". This means that you must use similar materials if you have hopes of them adhering to one another. Yes there is mechanical adhesion, but you would prefer to have both mechanical and chemical adhesion. Before you start the repair, find out from the manufacturer what the tank is made of. Polyethylene and Polypropylene are the hardest to adhere to so you may want to flame treat the surface first (google will explain). Great article and having struggled with the science of adhesion my whole career, before you attempt the real repair, as this article suggests - PRACTICE FIRST

Posted by: aschoenberg | November 24, 2018 5:20 AM    Report this comment

Very old ( 20+ years) tanks will become brittle and will not repair as seen here. Most will break apart just moving them around during removal. Replacement of same is recommended. Also the clear poly won't take as much of a beating as the black poly will.
Just an Old Captain

Posted by: Captain Rhan | November 23, 2018 7:54 AM    Report this comment

Great article. It gives the finer points of plastic welding. I did some welding on my own, but now I will add the reinforcement. That should last much longer. I noticed it is best to use similar plastic material, as some plastics do not adhere to other type. Thanks for the article.

Posted by: Henk | November 22, 2018 6:07 PM    Report this comment

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