Phil,

I guess that Photoshop also uses layers. My Photopaint (from Corel, yes) is obsolete.

Maybe the following technique can be helpful : First make a template with a circle and the appropriate background glow. this wil be the background for the sketch and should remain untouched during the whole process. Import your scanned sketch as a separate object or layer in front of the template. Stretch the layer to cover the circle. Now there should be a ruler to change the opacity of each object/layer. Set the scanned sketch layer at about 25% opacity. Now the layer will look transparent. Instead of working on this layer, create a new layer on which you copy (not really copy and paste, but draw from skratch) the stars as seen on your scan. Repeat this process for a 'nebulae' layer, a 'halo' layer, a 'glare' layer. Once all layers are rendered, delete the scan layer. Now ajust opacity for each layer according to your taste and combine them with the background for the final drawing. Never use a dark color to darken a lighter feature, use the eraser tool instead. The dark color will show up on the final sketch as an artefact, while the eraser will make the feature more transparent.
I hope this whole process makes sense to you? Your scan only serves to anchor the elements on the other layers. Any defects on the scan don't interfere cause the scan will be deleted anyway.

Clear skies,