Water tight printing

What's the trick to printing something that is water tight?

Extra solid layers on the top and bottom? High infill?
I printed a shot glass with my default settings and, even though it looked good, I was surprised to find that it slowly leaked water (very slowly mind you, I didn't notice until the next day)

I would also love to hear advice from people who know how to do this!

I am getting "reasonable" prints now; still a lot of tweaking to do before I would say I am really happy, but my main gripe is that the bottom and top layers are nowhere near watertight. I have been printing some 20 mm calibration cubes - overall dimensions are close to spot-on, and everything I print now clearly resembles the object it is intended to be - yay! I have been tweaking my Skeinforge / Pronterface settings, but I suspect I have a fundamental error or incompatibility in my settings somewhere.

The bottoms of the blocks are "formed" correctly on the raft (3 layers of closely spaced parallel diagonal lines; alternate layers at 90 degrees to each other), but the filaments that are laid down are not quite dense enough to completely "fill" the space - it is as if the filaments are slightly too thin (moving the extruder too fast for the extrusion rate?), and / or slightly too far apart.

The main build process goes OK - perimeter shells formed, and core fill placed. The side face shells are not too bad (IMHO), and seem to be nice and dense. I need to do some work to get the core fill and the surface shells perfectly aligned, but all in all, not too bad.

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Follow my Mendel Prusa build here: [julianh72.blogspot.com]

I have printed a 50mmx50mm box that is watertight, I made the bottom 5mm thick and the side walls 3mm thick. I use 3 solid layers on the top and bottom always, however since changing my hobbed insert I am getting slippage that I can't fix at the moment meaning even my solid layers are not solid like they used to be

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Experimenting in 3D in New Zealand

I may have inadvertently found at least half the "fix" to my print quality issues:

Without tweaking any firmware or software settings, I have found one thing that seems to make a dramatic improvement to the quality of my prints is using genuine Scotch Blue Tape 2090 on my print bed.

If you check out my blog posting on the subject [julianh72.blogspot.com] , you can see that printing on Blue Tape gives me MUCH improved resolution overall: on the bottom face, the side faces, and the top face. I really wasn't expecting such a dramatic change to the whole print quality, just from having a better bond between the bottom printed layer and the print bed.

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Follow my Mendel Prusa build here: [julianh72.blogspot.com]

But the main cause of my printing problems turned out to be backlash - thanks to Triffid Hunter for pointing out the symptoms to me, and what to do about fixing it.

After tightening my belts, my print quality has improved dramatically: [julianh72.blogspot.com]

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Follow my Mendel Prusa build here: [julianh72.blogspot.com]

I've done a little experimentation and found a good formula that works for me.

0.35 infill
4 fully filled layers top and bottom
4/3/3 on the perimiter settings
No fan under any circumstances!

Please try it and let me know if it works for you.