Finishing white painted furniture

Hi ...Is there a forum here on painting ....specifically after information on finishing (and sealing) furniture painted with water based paint? Been applying (Cabots w/b ) polyurethane but making a mess of it and too much sanding required (I think I'm losing a lot through sanding as my application is usually bumpy...need advice ... what type of roller is best, should I thin or can I just spray on some varnish if so, what's cheap...etc ...). Much of the furniture is white so whatever I do it needs to be non yellowing. Any advice would be appreciated. Cheers:?