Cut a patch from a shower curtain and use PVC pipe cement from your dad's garage to patch a leak in a PVC air mattress, inflatable boat, dry bag, etc.
Garbage Santa will reward you for learning this skill.
He'll put all kinds of nice inflatable stuff out at the curb for you.
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Step 1: Inflate
Air mattress inflation blowers are now superabundant in thrift stores in the U.S. I think I paid $2 for this one that comes with alligator clips for 12 volts.
Big airmattresses often have a big deflation cap and a smaller inflation valve. I usually use the big cap.
One handy trick to know is the mattress inflates a lot faster if you blow the air AT the big intake instead of IN the intake. That's because the air jet from your blower entrains a bunch of surrounding air and sucks it into the mattress along with it.
Step 2: Cap It
When the mattress is puffed up enough I shove my thumb in the intake and bend it to puff up the thumb muscles. That seals it while I hunt around for the cap. Usually there's a rubber plug with a lip and a screw cap to go over that.
Because the air mattress has so much surface area only a few psi are needed to inflate it properly (or pop it). (pounds per square inch * square inches = many pounds)
Next we'll find the leak. One way is to paint soapy water all over it and look for big soap bubbles. It's a sunny day so I'm going to take it swimming and look for bubbles in the water.
Step 3: Go Swimming and Find the Leaks
I put on my wetsuit and jumped into the lagoon with the aircraft carrier and spy ships to find the leak.
This is the first sunny day after a lot of rain so the sun's glare is blinding me.
Also my nordic cave-fish ancestors never got exposed to direct sunlight and evolved away their tolerance for it.
People who can be exposed to sunlight without getting sick don't need to make up stories about being the "master race".
Step 4: Leaks Found!
I had some trouble in the lagoon. The mattress was so full of air I had trouble pushing much of it underwater to look for bubbles. Then the wind blew me away from shore. Fortunately the leaks were big enough that I was eventually able to sink the mattress and see bubbles.
I wiped an area near each leak sort of dry and drew arrows pointing at the leaks. A sharpie marker is good for this because it can mark a surface that's slightly damp. Enough for a couple of arrows anyway.
Now I'm ready to patch. I hung the mattress in the sun to dry out and got my mask out.
It's got a fresh organic vapor cartridge in it so I won't have to breathe or smell any of the braindamage fumes from the glue.
I cut a couple of patches from another air mattress that was slashed beyond repair by a store that threw it away.
Step 5: Rough Up and Cut Out Patches
Use sandpaper to rough it up.
If your patch comes from a shower curtain it will be greasy and you'll have to de-grease it first.
If the mattress or patch are new you'll also need to degrease them by scrubbing with detergent. Plastic things come coated with some kind of oily stuff to keep them from sticking together in transit.
You've de-greased it enough if water sheets out on it. If the water beads up, degrease it some more. You can also do that by wiping it off with alcohol or the nasty greaseless solvent of your choice. Not Gasoline. It's oily.
SOURCES OF PATCH MATERIAL
My patches came from a totally shot air mattress.
In the past I've cut patches from shower curtains and a raincoat.
Any thin flexible sheet of PVC will be fine.
PVC won't stick to urethane and vice versa. In the U.S. most flexible plastic things are PVC.
That may be different in Europe due to health regulations.
SHAPE AND TAPER
Rounded patches are easier to stick down. If a patch has corners they tend to peel up.
In the past I've tried sanding the edges thin so they would taper down and stretch with the surrounding material better. That's worked well but usually isn't necessary.
Step 6: Rough Up Around the Leaks
I'm using sandpaper to rough up the area around the leaks. One of the leaks is in the top surface which has a flocked fibery surface. I sanded that until most of the fibers were gone and I was down to the plastic.
I didn't need to de-grease because this thing is old enough that the manufacturing oil has worn off already. Water sheeted out on it when I had it in the lagoon looking for bubbles.
Step 7: PVC Cement
These instructions are for how to patch using regular PVC pipe cement such as your dad has in his garage. If you can find some that says "flexible" on the label somewhere, that's good. If there's some dried glue on the lid, it should be a bit chewy instead of brittle.
If you're shopping for new glue, you can't beat this "HH-66" stuff for PVC. It's made for truck tarps and ilk. The directions are much different though than using pipe cement. You let it dry first, then re-activate your patch with heat.
Follow the instructions on the PVC cement you have. Some types recommend wiping with MEK or other solvent first. The solvent helps by making the plastic get soft and expand a little.
The cement has a brush attached to the underside of the lid, which always makes me happy.
Don't breathe any of these fumes. If you smell them your brain just died a little. Use your mask.
Wear gloves and keep it off your skin.
Step 8: Apply Glue
Paint the glue on the patch and on the area around the leak.
By the way, these methods and materials are not good for patching bicycle innertubes.
Get a proper bicycle patch kit for that. They only cost $2 or so.
Step 9: Apply Patch While Wet
Stick the patch down while it's still a bit wet. Squish it down hard and work the bubbles out.
Here I'm using my knuckle and the face of a hammer to rub and squish.
Step 10: Clamp It
To make extra sure, I'm using C-clamps (G-Clamps in other countries) to maximize contact while things set up. You could also stack some books on it or whatever is handy.
After an hour or so I took the clamps off and put the thing in the sun to drive off the braindamage solvents.
And that's it! It holds air and works fine!
If your patch leaks you can heat it up with an iron or hot air gun and squish it down. The melting point of the cement should be slightly lower than that of the sheet materials.
If that doesn't work, you can either peel it off and start over, or patch the patch.