Splinter - the poly I use takes at least half and hour before it starts to go off so there is nor time problem.  I don't thin it down or retard it at all, if I did I think it would then take too long to start to go off.  I'll post some small decorated boxes I've been make recently where I've 'set' eggshell and pau shell into wet poly - with those I sometimes wish the poly would go off quicker so I don't disturb the bits I've already applied, when put the adjacent bits on........