Pinewood Derby - Polyurethane as Wood Sealer?

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I'm preparing for a Pinewood Derby for the young boys at our church, and getting supplies together, so I can help those boys that may not get the support at home, to build their cars. I'm also going to build an "outlaw" car myself, just for kicks and giggles.

I was about to go get some wood sealer, but it occurred to me that I have a can of polyurethane finish, and was wondering if this would work as a wood sealer, in preparation for painting the cars. Also, can enamel paint or acrylic enamel paint be painted over the polyurethane, with proper adhesion and no reaction? Would a 2-3 day dry time between applying the polyurethane and applying the paint over the top, be sufficient to avoid issues?

If the polyurethane won't work, I have no problem getting some wood sealer. But I have already spent quite a bit, getting specialty Pinewood Derby tools and supplies. (My wife thinks I'm crazy for it.) And I hate waste, so I don't like the idea of buying a good sized container of wood sealer, that we will only use a very small portion of. Then it may sit on the shelf for years, until I realize it has went bad, and throw it out.

Anyone with any experience on this subject? Please share.
 
I always put the polyurethane over the paint. I'm not sure that paint would adhere to polyurethane. Why do you need wood sealer? I'd just sand and paint the wood. My son's car won the troop competition race. Certainly didn't win on looks. LOL
 
When I did our cars years ago I went to a decent hobby shop and bought 24 hour set epoxy NOT the 5 minute stuff. Mixed up a batch on a dixie cup used a heat gun or hair dryer on hot aimed into the dixie cup and used a cheap throw away brush and brushed onto the wood. When heated the epoxy will thin out making it look like glass when cured and smooth. A little goes a long way when heated. The heated epoxy will make it harden in about 3 hours. I did this when building plywood 1/4 scale gas hydroplanes in the stringers and bulk heads before I put on the top decking making it sealed and super strong. Take picts of the pinewood car when finished.
 
Good job helping the kids who don't have help at home. Awesome. (y)

As mentioned above test on a piece of scrap wood what you want to do.

I won Pinewood Derby about 100 years ago. Dad and I spent a lot of time making sure the car went straight, not so much on looks. Still have that car somewhere.

When my son was in Cubs I was Cubmaster for a short time. I built a car that looked like an Indy car so it looked fast but I didn't tune it for speed. Quite a few boys beat my fancy car, they loved it! Great times.
 
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I always put the polyurethane over the paint. I'm not sure that paint would adhere to polyurethane. Why do you need wood sealer? I'd just sand and paint the wood. My son's car won the troop competition race. Certainly didn't win on looks. LOL
From everything I have studied, and from personal experience, if paint is applied directly to wood, without a sealer, the wood absorbs the paint. It will take several coats of paint to seal wood. And you don't get the smooth glass like finish. Sealing the wood first, seals up all the pores in the wood, so the paint is being applied to the smooth sealer instead of the wood, giving a very nice finish.
Why not test it on a piece of scrap wood, if you have time? I would apply the polyurethane, wait a day or two, then apply the paint. Then let it dry and then put some tape on the paint and see if the tape pulls the paint off.
I was thinking along these same lines. Good idea. I'm going to do exactly that.
 
Good job helping the kids who don't have help at home. Awesome. (y)

As mentioned above test on a piece of scrap wood what you want to do.

I won Pinewood Derby about 100 years ago. Dad and I spent a lot of time making sure the car went straight, not so much on looks. Still have that car somewhere.
I sure enjoyed doing Pinewood derby with my own boys, many years (decades) ago. It was always fun, for me and my sons. I'm hoping I can make it as much fun for these kids.
 
The most important things to remember are to keep the weight in the front, get as close as you can to 5oz without going over, sand the wheels smooth, graphite on the axles, and epoxy the axles in.

Also, make sure the Pack committee plans on giving consolation prizes to all the cubs. They are overjoyed with the recognition. :)
 
The most important things to remember are to keep the weight in the front, get as close as you can to 5oz without going over, sand the wheels smooth, graphite on the axles, and epoxy the axles in.

Also, make sure the Pack committee plans on giving consolation prizes to all the cubs. They are overjoyed with the recognition. :)
Interesting, it was always a thing to keep the weight as far back as possible, still seems to be:
Do you have a secret nobody else knows?


Going to get mixed reaction to everyone gets a prize. Lots of debate.
 
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Interesting, it was always a thing to keep the weight as far back as possible, still seems to be:
Do you have a secret nobody else knows?


Going to get mixed reaction to everyone gets a prize. Lots of debate.
Seems that when the weight is in the back, the car is more likely to jump out of it's lane. That's my experience.

Bigger prizes for 1st 2nd 3rd, participation ribbons or patches or something for everyone.
 
The only thing that I can add is that these cars tend to have mis-aligned axles causing them to rub against one side of the center guide strip. A little testing and alignment of the cross axles tends to increase speed.
 
The only thing that I can add is that these cars tend to have mis-aligned axles causing them to rub against one side of the center guide strip. A little testing and alignment of the cross axles tends to increase speed.

Not a problem. Pinewood speed parts. (y)
 
get as close as you can to 5oz without going over, sand the wheels smooth, graphite on the axles
When we (well, my Dad) built cars, he cut a slot in the bottom, filled it, then filled more so that it extended out of the wood. Weigh it and then file off lead to reach the max weight. Sandpaper'd the "axles" on the ends where the wheels sat, get them as square as possible to each other, and last (I think this is not allowed anymore), shape the wheels to have a single high edge (not sure how to describe that). This reduces the contact area to the absolute minimum.
 
The most important things to remember are to keep the weight in the front, get as close as you can to 5oz without going over, sand the wheels smooth, graphite on the axles, and epoxy the axles in.

Also, make sure the Pack committee plans on giving consolation prizes to all the cubs. They are overjoyed with the recognition. :)
I've heard many theories on weight distribution. Some, like you, believe the weight should be biased towards the front. Others like the cg to be towards the center of the car. But by far, the majority of what I read suggests that it should be biased towards the rear of the car, somewhere between .75" and 1" forward of the rear axle.

I've even seen articles that discuss how the weight distribution reacts to different parts of the track. That is, best placement during the descent vs best weight placement on the flat vs best overall.

Curious, what are you basing your weight forward opinion on?
 
Man, this thread take me back. I was once a cub scout, though I'll admit I never really cared for it. Even as a kid, the whole operation gave me an uneasy vibe.

Anyway, I'll never forget the one Pinewood Derby I built a car for. Full disclosure: it was TERRIBLE. My level of disinterest trumped any sense of style or engineering. Accordingly, my car performed horribly.

Silver lining of the story? I ended up winning the trophy for "strangest car" or something of the sort, a trophy that was as big if not bigger than the 1st place job. I couldn't have been more proud of myself, at the time.
 
Helped my son build several of these. The fastest one we made I called 'Box Stock'. I just cut the box the kit came in to the shape of the wooden block and glued it on. I think he might still have it i n his 'Treasure Box' somewhere.

Looked a lot like this,

pinewood derby.jpg
 
I've heard many theories on weight distribution. Some, like you, believe the weight should be biased towards the front. Others like the cg to be towards the center of the car. But by far, the majority of what I read suggests that it should be biased towards the rear of the car, somewhere between .75" and 1" forward of the rear axle.

I've even seen articles that discuss how the weight distribution reacts to different parts of the track. That is, best placement during the descent vs best weight placement on the flat vs best overall.

Curious, what are you basing your weight forward opinion on?
Seems that when the weight is in the back, the car is more likely to jump out of it's lane. That's my experience.
 
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