solid layers over low infill.

causes all sorts of problems, it can sometimes take 5 layers to get a nice solid layer, depending on the thickness of your layers.
not only does it leave holes but it can also drag through when the filament has curled up where it's broken.
maybe if it's calculated as bridges it would help?

in the past I've sometimes cut and spliced different fill amounts to get a higher fill pattern ontop of a lower one before a solid layer which helps.

I use a simple rule that mostly solves this. Keep the solid layers equal to or greater than .7mm thick. This means at 0.25 layers you have three, but at .1 layers you need seven layers. At 0.05 layers you need 14 its that simple. This makes even more sense when you consider you want it equal to the outer shell thickness which is at least 1mm if using two shells and a 0.5 nozzle or at least 0.7 thick if using a 0.35 nozzle and 2 outer shells.

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FFF Settings Calculator	Gcode post processors	Geometric Object Deposition Tool Blog
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Also your filament shouldn't be snapping. You must be stretching it too much.

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I guess I could splice in a slower speed for the solid layers...
how else would I change the 'stretching'?
extrusion width is set to automatic, if anything I'm over extruding slightly. temp? up? down?

Another work around if you are using a firmware capable of real time speed modification like Marlin. Just find the layer in the Gcode you want to go slower and add "M220 S50" at the beginning of the layer to reduce the speed to 50% and then "M220 S100" at the end of the layer so it goes back to 100% speed. Change the S number to what ever you would like to make it go faster or slower.

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FFF Settings Calculator	Gcode post processors	Geometric Object Deposition Tool Blog
Tantillus.org	Mini Printable Lathe	How NOT to install a Pololu driver

oh duh. there is an option for the solid layer speed in slic3r
I'll try 50mm/s for that instead of the 100 I've been doing.

some improvement at 50mms but not a huge amount. this was on rectilinier at 0.15 fill
0.2mm layers infill every 2 layers (so infill was 0.4mm... were the solid layers 0.4 or 0.2?...)

I might print that bridge calabration test thing and see how that comes out.