In the hydroelectric industry, we used to get called upon by Belzona sales people regularly. They claimed to have products to repair or improve very nearly anything.
One issue we were constantly confronted with was cavitation erosion on the runners ("turbine wheels") and some of the stationary parts in the flow passages such as stay vanes and wicket gates. During an outage, we'd typically identify the worst of the cavitation damage, and repairs would be made. These repairs typically consisted of gouging out the cavitated areas (which looked like weathered concrete, even though they were either a stainless overlay weld or original medium carbon cast steel). Once the gouging down to sound steel had been done, it was ground smooth and blended with smooth transitions to the original metal. Overlay welding consisting of several layers of E 309L stainless followed by a proprietary stainless alloy (from the turbine builder)- a stainless with added cobalt followed. The finished weld repairs were then ground and polished to match the correct contours of the surrounding metal.
The Belzona representatives saw a chance to sell some of their products. One other engineer bit on the idea and he was in a position to make that decision. The mechanics, who'd seen many years of outages and cavitation repair, dismissed the Belzona claims as "Bondo" or "high priced JB Weld". Setting their misgivings aside, the Belzona was applied instead of weld buildup to some test areas of cavitation damage. A year later, during another scheduled outage, an inspection was made. Interestingly, some of the "Sharpie" marker notes and lines on the runner buckets and stay vanes and wicket gates was still partially visible. The Belzona was gone, not a trace remaining. Naturally, the Belzona rep was on hand to see how their product worked. Seeing the Belzona had vanished, before we could say a word, the rep was stating: "You failed to prepare the substrate properly- had you done so in accordance with our specifications, the Belzona would have held up in service".
Belzona's idea of correctly preparing the substrate is to abrasive blast with a fairly coarse abrasive down to "white metal". We could not do that inside the turbine, so ground the areas to clean metal and roughed them up with air needle scalers and chipping guns with chisels- all under the watchful eye of the Belzona rep. We maintained temperature requirements and the Belzona had appeared to have set and hardened well. The Belzona rep called his regional people, and we got the same song and dance. It was the last time we ever entertained the notion of using Belzona, and the name became kind of a joke around the powerplant.
Setting the experience with the turbines aside, we did use similar products from Belzona's competitor, DevCon. We were never again crazy enough to try anything like it inside the turbines. However, for odds and ends of little repair jobs, we had fairly good luck with the DevCon. On one occasion, we had an air compressor cylinder head with heavy corrosion damage (in a damp underground hydro plant run remotely, this was a given). One of the mechanics used a DevCon resin with powdered bronze, the only stuff we had at the time. The area to be repaired was where one of the valves seated in the head (a disc type valve assembly that sat on a copper ring gasket). That repair held for well over a year until the next outage when we replaced the cylinder head. Occasionally, on smaller low pressure centrifugal pumps, handling cold water, we'd use the DevCon with the bronze, and usually got good results for a limited time period. We never looked at any repair resin as a permanent fix. I think that is the key to using this sort of product, aside from being realistic in matching the repair resin to the application.
The classic emergency repair story concerns my brother and one of his automobiles. As a young prosecuting attorney out in Wyoming, my brother got it into his head to buy a new Jaguar. This was many years before the internet or personal computers, so buying a Jaguar meant a trip to the dealers in either Rapid City, SD, or Denver, Colorado. Calling the dealers and getting prices, my brother reasoned that the Jag cost a bit more because it had to be shipped inland from the port of entry. He called the Jaguar dealer in Brooklyn, NY- where our parents still resided. The Jag dealer in Brooklyn gave a much lower price quote. The result was my brother ordered the Jag from the Brooklyn dealer. The dealer sent the paperwork with the VIN out to my brother so he could register the car in Wyoming. A family friend was happy to drive the car out to visit my brother and fly home again. A few days before the friend was due to pick up the Jag and head west with it, Dad got a phone call from the dealer claiming they'd "made a mistake with the VIN". Paperwork had to be re-sent out to Wyoming and a new registration and title made out. This was finally done with, and our friend hopped in the Jag and headed to Wyoming without incident.
Some months after my brother had been driving his new Jaguar, he started noticing increasing amounts of oil under the car on his garage floor. Curious, he put clean cardboard under the car's oil pan (or sump as it might be called in the UK ?). Sure enough, the car's oil pan was seeping oil, and not anywhere near the drain plug or any gasketed joint. The oil was seeming to be seeping thru the cast aluminum oil pan. My brother got a screw driver and a light and got the car up on jackstands. Poking around the oil pan, he literally struck oil. He discovered a "repair" to the oil pan had been made using some epoxy resin, what looked like cat litter, and some silver paint. There was a hell of a porosity in the casting, right from the factory. The Brooklyn dealer, knowing the car would be going on a one-way trip to Wyoming, "fixed" the leak in the oil pan. Who knows whether the Brooklyn dealer invoiced the Jaguar company for replacing an oil pan on that car and pocketed the money. Suddenly, the last minute "mistake" with the VIN became apparent as being no mistake. The dealer saw a way to get rid of a problem by exchanging one XJ 6 of the same color and optional features for another just like it.
That left my brother with a problem. He called the Jag dealer in Rapid City and learned that while Jaguar would eat the repair as warranty work, getting the car there and associated expenses were on my brother's dime. So, my brother drained the oil in his Jaguar, got some solvent and JB weld and attempted another "repair" on the oil pan. He could not get a good repair, so loaded the trunk of the Jaguar with a couple of cases of oil and set out for Rapid City. He said he left a trail of oil drops from Gillette, Wyoming to Rapid City, South Dakota and was stopping every 50 miles or so to check oil level and throw in more oil.
In a kind of repeat performance, we had a secretary on one jobsite who was a real party animal. She arrived late to work one morning, claiming she had been out the night before (at that time, finding secretaries to work on a construction site for short durations was difficult, so we took whom we got). She claimed she had to hitch a ride into work as her car was off the road a few miles back. Asked why, on an otherwise fine spring morning, her car was off the road, the story came out. Seems she was feeling the effects of the night before, and claimed a squirrel ran into the road in front of her car. With sluggish reaction time, she succeeded in locking up the brakes and skidding off the road. OK, we said we'd get a chain and a truck and go get her car. Not so simple, she said. She said her car had fetched up on a rock which had stoved a hole in the oil pan and there was oil all over the place. She asked if we'd pull her car off the rock and get it to where she could work on it. A couple of obliging fellows got her car pulled off the rock and up on the shoulder of the road and left it for her.
Our secretary went to another contractor's trailer and got their secretary into the matter. The two secretaries got nail polish remover and JB weld and more oil during their lunch break. They cleaned the oil pan with the nail polish remover, and stuck a wad of JB weld onto the oil pan and into the dent and hole the rock had made. Next morning, the secretary caught a ride to her car, dumped in fresh oil, and drove it into work- on time for a change. She ran the JB Weld repair for a few weeks until she got a couple of paychecks and could buy the parts to fix the car properly with her father's help. JB Weld is the stuff of legends and jokes, and in most of the auto parts stores up this way, there are some displays on the counters of JB Weld and what it can bond and repair.
My attitude towards repairs, whether done with Belzona, DevCon , or JB Weld, is that they are a stopgap measure or emergency repair to get a person home or run equipment until it can be taken off line for a proper repair. Belzona was (and may still be) about the priciest repair resin out there. For a few years in the 1980's, it seemed that many of the Belzona salesmen were recent graduates of the merchant marine schools. With the US merchant marine being very limited, engineer graduates were hard pressed to find jobs in shipping. They'd wind up coming to powerplants to preach about the wonders of Belzona.
On another level, we used to get salesmen trying to sell repair electrodes- welding rods they claimed would weld any ferrous metals and anything short of a broken heart or the crack of dawn. One particularly enterprising salesman (Certainium, or MG Messer, I forget which) would appear with a folding hand held welding shield and ask for some different types of steels to be welded together. Up at the fleet garage at the powerplant, the mechanics would fetch a busted truck spring leaf, a knife from a wood chipper, and a chunk of A-36 heavy angle or channel. The salesman would then ask them to coat the surfaces to be welded with grease, so the mechanics would oblige. The salesman, in his shirt and tie, would take out a pair of welding gloves and unfold his hand-held shield, and ask to use a stick welding power supply. He'd proceed to weld the junk together, the arc burning through the grease and stinking and smoking up the fleet garage. After he'd welded the different steels together, he'd ask the mechanics to try to break the welds. They'd put the junk he'd welded in a heavy vise and attack it with a 16 lb sledge. Usually, the steel adjacent to the welds bent or fractured while the welds held. It was an impressive demonstration, and of course, the guy then wanted to sell a few boxes of his super-duper miracle repair electrodes. Our employer was in a position to afford to buy a box or two of these repair electrodes, but the price to a small shop or someone like a farmer or logger would be exorbitant. These salesmen would then attempt to convince us to use their electrodes on turbine repairs and structural work. We'd ask for some documentation on the electrodes like AWS specs, melt analysis, and similar. All we'd get is: "It's proprietary". We'd explain that we did any welding on the turbines, structural work, piping and similar using pre-qualified weld procedures with welding filler metals of known compositions and known physical properties. The salesmen would then show us the specs- such as they were- as to how their miracle repair electrodes were way stronger than the usual welding filler metals (ER 70 XX, E 309L, E 8018, etc). We never could get any real documentation on these miracle repair electrodes, so they were relegated to the fleet garage. As we were to learn, these miracle repair electrodes DID live up to the salesmens' claims in many regards, but the weld deposits were usually quite hard, and there was so much chromium and nickel in those electrodes that we could not cut thru the welds with an oxyacetylene torch (when we used those electrodes to weld grousers to bulldozer tracks for winter and had to remove them come spring).
I tend to go with the old adage: "If it seems to good to be true... it probably IS to good to be true". I also have spent enough years in my craft and profession to disbelieve that a repair resin will ever take the place of good metal.
In another instance, I was asked, as a consulting engineer, to take a look into why a large shovel used in surface mining operations, had "run away" when being moved up a temporary ramp. The runaway was such that this huge electrically driven shovel lost driving power to one of its tracks, and became uncontrollable on the ramp. The mine operations people had put a D-9 'Cat behind the shovel to trail it up the ramp, knowing the ramp was a bit beyond the performance envelope for the shovel. The shovel, when it ran away, spun around, pushed the D-9 aside like a toy, and demolished a stake-rack truck which was following to tend the power cable. One man in the truck was killed on the spot. The investigation revealed some badly worn clutch teeth and other badly worn parts. As the Belzona youtube describes, the mine in the case of the runaway shovel also knew the cost of downtime for the shovel and simply did not want to take it out of service for proper repairs. Clutch teeth on one of the clutches to drive the tracks were badly worn beyond the point of any adjustment, and other parts were also badly worn. Seeing the size of that shovel and the kind of torques and forces developed, there is no way I would believe, let alone want to trust, a Belzona repair in that kind of application.'
It is one thing to parge on the Belzona if you are repairing a centrifugal pump casing, but quite another to repair something like drivetrain parts in a large mining shovel or dragline. These sorts of products, like the miracle repair electrodes, have their place and uses. In my opinion, the trick is to recognize the limitations of these sorts of products and set aside the claims the vendors make for them.