Homebrew PCBs

Hi,

I'm using an HP LaserJet 1300 set to "Dark" on 600dpi to print simple 
PCBs (10mil or greater widths).

When I look at the masked areas under a microscope after etching with 
sodium persulphate, I see a lot of porosity, i.e. little holes etched 
in the black areas. I'm ironing quite heavily and the boards look very 
solid when inspected under the uscope after the backing paper has been 
soaked off so I don't think that this is an adhesion issue....

Is there anything that can be done to make the ink less porous? I note 
that http://www.pulsarprofx.com/ have what they call a green "TRF" that 
is supposed to do this - anyone know what it really is?

Cheers

Nick