graphics - World plot without borders between countries

I would like to plot the world map without the countries' borders and just the coastlines (in Mathematica 8). I need to overlap a temperature contour plot on the world map.

When I plot country data with lines, it is so crowded because of the lines related to the borders. Plotting with polygons does not help, since I cannot overlap the second plot on top of it.

For example in the following command how can I add an option to exclude the borders?

    Graphics[CountryData[#, "Polygon"] & /@ CountryData["Continents"], 
             ImageSize -> 400] /. Polygon -> Line

In other words, I need something nice like this but not with polygons and only the coastlines:

    map = Graphics[{GrayLevel[0.5], AbsoluteThickness[1.6], 
   CountryData[#, {"FullPolygon", {"Equirectangular", {0, 
       180}}}]} & /@ {"Antarctica", "World"} /. Polygon -> Line, 

   Frame -> True, FrameStyle -> Directive[Thick], 
   PlotLabel -> "Precipitation", 
   FrameLabel -> {"Longitude (°E)", "Latitude (°N)"}, 
  Axes -> True, AxesStyle -> Directive[Gray], 
  LabelStyle -> Directive[Black, 18, Bold], PlotRangePadding -> None,
  ImageSize -> 600, 
  GridLinesStyle -> Directive[AbsoluteThickness[2], GrayLevel[0.25]],
  GridLines -> {Automatic, Automatic}, 
  Method -> {"GridLinesInFront" -> True}];

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