Hi gc123
I've never tried Ultrasun but I think I would either apply it one hour after the oestrogel and keep an eye on those specific areas in case you need to reapply (ie if not as effective as on clean skin) Or I'd use ordinary sunscreen on those areas and keep reapplying as you would with the ordinary stuff.
The other alternative, if its just for holidays, is to switch your gel application time and maybe put on your gel after your early evening/post sunny day activities. Lots of people use gel in evenings rather than mornings as it suits their routine. Personally I'm fortunate not to need very regular gel 'fixes' so I vary my application to suit my daily activities if I'm doing something where my morning application doesn't suit.
If your question is holiday related - have a wonderful time 😎
Suzie