Dutch,
I have done inside soldering with any alloy from Tix (melting temp down at 275 F) and up to high silver / gold content braze alloys that melt at 1550 F. My rules for soldering:
1) Absolutely clean surface....no dirt, no grease, no OXIDES of any kind.
2) Appropriate flux on the joint surfaces and surrounding area.
3) Proper placement of the soldering / braze alloy at the joint. (Sometimes you need to warm / heat the part to be joined until any liquid in the flux boils off....otherwise the bubbling of the flux may / will push the pre-placed solder / braze alloy away from the joint.
4) As Wade says, the solder / braze alloy will ALWAYS run toward the heat. Always, if possible, heat from the opposite side of the joint to get the alloy to flow through it toward the heat.
5) If the joint is not going as planned, STOP !!! Re-clean and start again. Most botched solder / braze jobs are from applying heat in the wrong place (i.e. directly on the soldering / braze alloy) or heating the joint area too fast and to too high a temperature which will often "burn" the flux and $#@* up the joint.
To solder a powder flask from the inside, for example, I would clean the joint area and then wire the parts together to hold the joint tightly closed. Place the joint down and put the solder / braze alloy inside above the joint. Heat gently from the outside until the alloy flows. With the joint down you will get the capillary action of the liquid alloy as well as gravity helping get the alloy where it needs to go. And, if you have sufficient alloy in the joint, you can draw it to dry areas of the joint just with the torch. The liquidus alloy will follow the heat. To go all the way around a flask, I would use tongs to move the flask orientation and add alloy as needed through the open mouth. This can also be done in two or more heats with re-cleaning and re-fluxing for each round of soldering.
Hope this helps.
PS A tight joint and capillary action are your friends insuring a clean, very strong joint and making it easy to get the solder to flow where you want it and not everywhere else.